/*
Module: rope


High-level text containers.

Ropes are a high-level representation of text that offers
much better performance than strings for common operations,
and generally reduce memory allocations and copies, while only
entailing a small degradation of less common operations.

More precisely, where a string is represented as a memory buffer,
a rope is a tree structure whose leaves are slices of immutable
strings. Therefore, concatenation, appending, prepending, substrings,
etc. are operations that require only trivial tree manipulation,
generally without having to copy memory. In addition, the tree
structure of ropes makes them suitable as a form of index to speed-up
access to Unicode characters by index in long chunks of text.

The following operations are algorithmically faster in ropes:
- extracting a subrope is logarithmic (linear in strings);
- appending/prepending is near-constant time (linear in strings);
- concatenation is near-constant time (linear in strings);
- char length is constant-time (linear in strings);
- access to a character by index is logarithmic (linear in strings);
 */




/*
 Type: rope

 The type of ropes.
 */
type rope = node::root;

/*
 Section: Creating a rope
 */

/*
 Function:empty

 Create an empty rope
 */
fn empty() -> rope {
   ret node::empty;
}

/*
 Function: of_str

 Adopt a string as a rope.

 Parameters:

str - A valid string.

 Returns:

A rope representing the same string as `str`. Depending of the length
of `str`, this rope may be empty, flat or complex.

Performance notes:
- this operation does not copy the string;
- the function runs in linear time.
 */
fn of_str(str: @str) -> rope {
    ret of_substr(str, 0u, str::byte_len(*str));
}

/*
Function: of_substr

As `of_str` but for a substring.

Performance note:
- this operation does not copy the substring.

Parameters:

byte_offset - The offset of `str` at which the rope starts.
byte_len    - The number of bytes of `str` to use.

Returns:

A rope representing the same string as
`str::substr(str, byte_offset, byte_len)`.
Depending on `byte_len`, this rope may be empty, flat or complex.

Safety notes:
- this function does _not_ check the validity of the substring;
- this function fails if `byte_offset` or `byte_len` do not match `str`.
 */
fn of_substr(str: @str, byte_offset: uint, byte_len: uint) -> rope {
    if byte_len == 0u { ret node::empty; }
    if byte_offset + byte_len  > str::byte_len(*str) { fail; }
    ret node::content(node::of_substr(str, byte_offset, byte_len));
}

/*
Section: Adding things to a rope
 */

/*
Function: append_char

Add one char to the end of the rope

Performance note:
- this function executes in near-constant time
 */
fn append_char(rope: rope, char: char) -> rope {
    ret append_str(rope, @str::from_chars([char]));
}

/*
Function: append_str

Add one string to the end of the rope

Performance note:
- this function executes in near-linear time
 */
fn append_str(rope: rope, str: @str) -> rope {
    ret append_rope(rope, of_str(str))
}

/*
Function: prepend_char

Add one char to the beginning of the rope

Performance note:
- this function executes in near-constant time
 */
fn prepend_char(rope: rope, char: char) -> rope {
    ret prepend_str(rope, @str::from_chars([char]));
}

/*
Function: prepend_str

Add one string to the beginning of the rope

Performance note:
- this function executes in near-linear time
 */
fn prepend_str(rope: rope, str: @str) -> rope {
    ret append_rope(of_str(str), rope)
}

/*
Function: append_rope

Concatenate two ropes
 */
fn append_rope(left: rope, right: rope) -> rope {
   alt(left) {
     node::empty. { ret right; }
     node::content(left_content) {
       alt(right) {
         node::empty. { ret left; }
     node::content(right_content) {
           ret node::content(node::concat2(left_content, right_content));
     }
       }
     }
   }
}

/*
Function: concat

Concatenate many ropes.

If the ropes are balanced initially and have the same height, the resulting
rope remains balanced. However, this function does not take any further
measure to ensure that the result is balanced.
 */
fn concat(v: [rope]) -> rope {
    //Copy `v` into a mutable vector
    let len   = vec::len(v);
    if len == 0u { ret node::empty; }
    let ropes = vec::init_elt_mut(v[0], len);
    uint::range(1u, len) {|i|
       ropes[i] = v[i];
    }

    //Merge progresively
    while len > 1u {
        uint::range(0u, len/2u) {|i|
            ropes[i] = append_rope(ropes[2u*i], ropes[2u*i+1u]);
        }
        if len%2u != 0u {
            ropes[len/2u] = ropes[len - 1u];
            len = len/2u + 1u;
        } else {
            len = len/2u;
        }
    }

    //Return final rope
    ret ropes[0];
}


/*
Section: Keeping ropes healthy
 */



/*
Function: bal

Balance a rope.

Returns:

A copy of the rope in which small nodes have been grouped in memory,
and with a reduced height.

If you perform numerous rope concatenations, it is generally a good idea
to rebalance your rope at some point, before using it for other purposes.
 */
fn bal(rope:rope) -> rope {
    alt(rope) {
      node::empty. { ret rope }
      node::content(x) {
        alt(node::bal(x)) {
          option::none.   { rope }
          option::some(y) { node::content(y) }
        }
      }
    }
}

/*
Section: Transforming ropes
 */


/*
Function: sub_chars

Extract a subrope from a rope.

Performance note:
- on a balanced rope, this operation takes algorithmic time;
- this operation does not involve any copying

Safety note:
- this function fails if char_offset/char_len do not represent
valid positions in rope
 */
fn sub_chars(rope: rope, char_offset: uint, char_len: uint) -> rope {
    if char_len == 0u { ret node::empty; }
    alt(rope) {
      node::empty. { fail }
      node::content(node) {
        if char_len > node::char_len(node) { fail }
        else {
            ret node::content(node::sub_chars(node, char_offset, char_len))
        }
      }
    }
}

/*
Function:sub_bytes

Extract a subrope from a rope.

Performance note:
- on a balanced rope, this operation takes algorithmic time;
- this operation does not involve any copying

Safety note:
- this function fails if byte_offset/byte_len do not represent
valid positions in rope
 */
fn sub_bytes(rope: rope, byte_offset: uint, byte_len: uint) -> rope {
    if byte_len == 0u { ret node::empty; }
    alt(rope) {
      node::empty. { fail }
      node::content(node) {
        if byte_len > node::byte_len(node) { fail }
        else {
            ret node::content(node::sub_bytes(node, byte_offset, byte_len))
        }
      }
    }
}

/*
Section: Comparing ropes
 */

/*
Function: cmp

Compare two ropes by Unicode lexicographical order.

This function compares only the contents of the rope, not their structure.

Returns:

A negative value if `left < right`, 0 if eq(left, right) or a positive
value if `left > right`
 */
fn cmp(left: rope, right: rope) -> int {
    alt((left, right)) {
      (node::empty., node::empty.) { ret 0; }
      (node::empty., _)     { ret -1;}
      (_, node::empty.)     { ret  1;}
      (node::content(a), node::content(b)) {
        ret node::cmp(a, b);
      }
    }
}

/*
Function: eq

Returns:

 `true` if both ropes have the same content (regardless of their structure),
`false` otherwise
*/
fn eq(left: rope, right: rope) -> bool {
    ret cmp(left, right) == 0;
}

/*
Function: le

Parameters
  left - an arbitrary rope
  right - an arbitrary rope

Returns:

 `true` if `left <= right` in lexicographical order (regardless of their
structure), `false` otherwise
*/
fn le(left: rope, right: rope) -> bool {
    ret cmp(left, right) <= 0;
}

/*
Function: lt

Parameters
  left - an arbitrary rope
  right - an arbitrary rope

Returns:

 `true` if `left < right` in lexicographical order (regardless of their
structure), `false` otherwise
*/
fn lt(left: rope, right: rope) -> bool {
    ret cmp(left, right) < 0;
}

/*
Function: ge

Parameters
  left - an arbitrary rope
  right - an arbitrary rope

Returns:

 `true` if `left >= right` in lexicographical order (regardless of their
structure), `false` otherwise
*/
fn ge(left: rope, right: rope) -> bool {
    ret cmp(left, right) >= 0;
}

/*
Function: gt

Parameters
  left - an arbitrary rope
  right - an arbitrary rope

Returns:

 `true` if `left > right` in lexicographical order (regardless of their
structure), `false` otherwise
*/
fn gt(left: rope, right: rope) -> bool {
    ret cmp(left, right) > 0;
}

/*
Section: Iterating
 */

/*
Function: loop_chars

Loop through a rope, char by char

While other mechanisms are available, this is generally the best manner
of looping through the contents of a rope char by char. If you prefer a
loop that iterates through the contents string by string (e.g. to print
the contents of the rope or output it to the system), however,
you should rather use `traverse_components`.

Parameters:
rope - A rope to traverse. It may be empty.
it - A block to execute with each consecutive character of the rope.
Return `true` to continue, `false` to stop.

Returns:

`true` If execution proceeded correctly, `false` if it was interrupted,
that is if `it` returned `false` at any point.
 */
fn loop_chars(rope: rope, it: block(char) -> bool) -> bool {
   alt(rope) {
      node::empty. { ret true }
      node::content(x) { ret node::loop_chars(x, it) }
   }
}

/*
Function: iter_chars

Loop through a rope, char by char, until the end.

Parameters:
rope - A rope to traverse. It may be empty.
it - A block to execute with each consecutive character of the rope.
 */
fn iter_chars(rope: rope, it: block(char)) {
    loop_chars(rope) {|x|
        it(x);
        ret true
    }
}

/*
Function: loop_leaves

Loop through a rope, string by string

While other mechanisms are available, this is generally the best manner of
looping through the contents of a rope string by string, which may be useful
e.g. to print strings as you see them (without having to copy their
contents into a new string), to send them to then network, to write them to
a file, etc.. If you prefer a loop that iterates through the contents
char by char (e.g. to search for a char), however, you should rather
use `traverse`.

Parameters:

rope - A rope to traverse. It may be empty.
it - A block to execute with each consecutive string component of the rope.
Return `true` to continue, `false` to stop.

Returns:

`true` If execution proceeded correctly, `false` if it was interrupted,
that is if `it` returned `false` at any point.
 */
fn loop_leaves(rope: rope, it: block(node::leaf) -> bool) -> bool{
   alt(rope) {
      node::empty. { ret true }
      node::content(x) {ret node::loop_leaves(x, it)}
   }
}

mod iterator {
    mod leaf {
        fn start(rope: rope) -> node::leaf_iterator::t {
            alt(rope) {
              node::empty.     { ret node::leaf_iterator::empty() }
              node::content(x) { ret node::leaf_iterator::start(x) }
            }
        }
        fn next(it: node::leaf_iterator::t) -> option::t<node::leaf> {
            ret node::leaf_iterator::next(it);
        }
    }
    mod char {
        fn start(rope: rope) -> node::char_iterator::t {
            alt(rope) {
              node::empty.   { ret node::char_iterator::empty() }
              node::content(x) { ret node::char_iterator::start(x) }
            }
        }
        fn next(it: node::char_iterator::t) -> option::t<char> {
            ret node::char_iterator::next(it)
        }
    }
}

/*
 Section: Rope properties
 */

/*
 Function: height

 Returns: The height of the rope, i.e. a bound on the number of
operations which must be performed during a character access before
finding the leaf in which a character is contained.

 Performance note: Constant time.
*/
fn height(rope: rope) -> uint {
   alt(rope) {
      node::empty.    { ret 0u; }
      node::content(x) { ret node::height(x); }
   }
}



/*
 Function: char_len

 Returns: The number of character in the rope

 Performance note: Constant time.
 */
pure fn char_len(rope: rope) -> uint {
   alt(rope) {
     node::empty.           { ret 0u; }
     node::content(x)       { ret node::char_len(x) }
   }
}

/*
 Function: char_len

 Returns: The number of bytes in the rope

 Performance note: Constant time.
 */
pure fn byte_len(rope: rope) -> uint {
   alt(rope) {
     node::empty.           { ret 0u; }
     node::content(x)       { ret node::byte_len(x) }
   }
}

/*
 Function: char_at

 Parameters:
  pos - A position in the rope

 Returns: The character at position `pos`

 Safety notes: The function will fail if `pos`
 is not a valid position in the rope.

 Performance note: This function executes in a time
 proportional to the height of the rope + the (bounded)
 length of the largest leaf.
 */
fn char_at(rope: rope, pos: uint) -> char {
   alt(rope) {
      node::empty. { fail }
      node::content(x) { ret node::char_at(x, pos) }
   }
}


/*
 Section: Implementation
*/
mod node {

    /*
     Enum: node::root

     Implementation of type `rope`

     Constants:
       empty   - An empty rope
       content - A non-empty rope
    */
    tag root {
        empty;
        content(@node);
    }

    /*
     Struct: node::leaf

     A text component in a rope.

     This is actually a slice in a rope, so as to ensure maximal sharing.
    */
    type leaf = {

    /*
     Field: byte_offset

     The number of bytes skipped in `content`
    */
    byte_offset: uint,

    /*
     Field: byte_len

     The number of bytes of `content` to use
    */
    byte_len:    uint,

    /*
     Field: char_len


     The number of chars in the leaf.
    */
    char_len:   uint,

    /*
    Field: content

    Contents of the leaf.

    Note that we can have `char_len < str::char_len(content)`, if this
    leaf is only a subset of the string. Also note that the string
    can be shared between several ropes, e.g. for indexing purposes.
    */
    content:    @str
    };


    /*
     Struct node::concat

     A node obtained from the concatenation of two other nodes
    */
    type concat = {

        /*
        Field: left

        The node containing the beginning of the text.
        */
        left:     @node,//TODO: Perhaps a `vec` instead of `left`/`right`

        /*
        Field: right

        The node containing the end of the text.
        */
        right:    @node,

        /*
        Field: char_len

        The number of chars contained in all leaves of this node.
        */
        char_len: uint,

        /*
        Field: byte_len

        The number of bytes in the subrope.

        Used to pre-allocate the correct amount of storage for serialization.
        */
        byte_len: uint,

        /*
        Field: height

        Height of the subrope.

        Used for rebalancing and to allocate stacks for
        traversals.
        */
        height:   uint
    };

    /*
    Enum: node::node

    leaf - A leaf consisting in a `str`
    concat - The concatenation of two ropes
    */
    tag node {
        leaf(leaf);
        concat(concat);
    }

    /*
    The maximal number of chars that _should_ be permitted in a single node.

    This is not a strict value
     */
    const hint_max_leaf_char_len: uint = 256u;

    /*
    The maximal height that _should_ be permitted in a tree.

    This is not a strict value
     */
    const hint_max_node_height:   uint = 16u;

    /*
    Function: of_str

    Adopt a string as a node.

    If the string is longer than `max_leaf_char_len`, it is
    logically split between as many leaves as necessary. Regardless,
    the string itself is not copied.

    Performance note: The complexity of this function is linear in
    the length of `str`.
     */
    fn of_str(str: @str) -> @node {
        ret of_substr(str, 0u, str::byte_len(*str));
    }

    /*
    Function: of_substr

    Adopt a slice of a string as a node.

    If the slice is longer than `max_leaf_char_len`, it is logically split
    between as many leaves as necessary. Regardless, the string itself
    is not copied.

    Parameters:
    byte_start - The byte offset where the slice of `str` starts.
    byte_len   - The number of bytes from `str` to use.

    Safety note:
    - Behavior is undefined if `byte_start` or `byte_len` do not represent
     valid positions in `str`
     */
    fn of_substr(str: @str, byte_start: uint, byte_len: uint) -> @node {
        ret of_substr_unsafer(str, byte_start, byte_len,
                  str::char_len_range(*str, byte_start, byte_len));
    }

    /*
    Function: of_substr_unsafer

    Adopt a slice of a string as a node.

    If the slice is longer than `max_leaf_char_len`, it is logically split
    between as many leaves as necessary. Regardless, the string itself
    is not copied.

    byte_start - The byte offset where the slice of `str` starts.
    byte_len   - The number of bytes from `str` to use.
    char_len   - The number of chars in `str` in the interval
          [byte_start, byte_start+byte_len(

    Safety note:
    - Behavior is undefined if `byte_start` or `byte_len` do not represent
     valid positions in `str`
    - Behavior is undefined if `char_len` does not accurately represent the
     number of chars between byte_start and byte_start+byte_len
    */
    fn of_substr_unsafer(str: @str, byte_start: uint, byte_len: uint,
                          char_len: uint) -> @node {
        assert(byte_start + byte_len <= str::byte_len(*str));
        let candidate = @leaf({
                byte_offset: byte_start,
                byte_len:    byte_len,
                char_len:    char_len,
                content:     str});
        if char_len <= hint_max_leaf_char_len {
            ret candidate;
        } else {
            //Firstly, split `str` in slices of hint_max_leaf_char_len
            let leaves = uint::div_ceil(char_len, hint_max_leaf_char_len);
            //Number of leaves
            let nodes  = vec::init_elt_mut(candidate, leaves);

            let i = 0u;
            let offset = byte_start;
            let first_leaf_char_len =
                if char_len%hint_max_leaf_char_len == 0u {
                  hint_max_leaf_char_len
                } else {
                char_len%hint_max_leaf_char_len
               };
            while i < leaves {
                let chunk_char_len: uint =
                    if i == 0u  { first_leaf_char_len }
                    else { hint_max_leaf_char_len };
                let chunk_byte_len =
                    str::byte_len_range(*str, offset, chunk_char_len);
                nodes[i] = @leaf({
                    byte_offset: offset,
                    byte_len:    chunk_byte_len,
                    char_len:    chunk_char_len,
                    content:     str
                });

                offset += chunk_byte_len;
                i      += 1u;
            }

            //Then, build a tree from these slices by collapsing them
            while leaves > 1u {
                i = 0u;
                while i < leaves - 1u {//Concat nodes 0 with 1, 2 with 3 etc.
                    nodes[i/2u] = concat2(nodes[i], nodes[i + 1u]);
                    i += 2u;
                }
                if i == leaves - 1u {
                    //And don't forget the last node if it is in even position
                    nodes[i/2u] = nodes[i];
                }
                leaves = uint::div_ceil(leaves, 2u);
            }
            ret nodes[0u];
        }
    }

    pure fn byte_len(node: @node) -> uint {
        alt(*node) {//TODO: Could we do this without the pattern-matching?
          leaf(y)  { ret y.byte_len; }
          concat(y){ ret y.byte_len; }
        }
    }

    pure fn char_len(node: @node) -> uint {
        alt(*node) {
          leaf(y)   { ret y.char_len; }
          concat(y) { ret y.char_len; }
        }
    }


    /*
    Function: tree_from_forest_destructive

    Concatenate a forest of nodes into one tree.

    Parameters:
    forest - The forest. This vector is progressively rewritten during
    execution and should be discarded as meaningless afterwards.
    */
    fn tree_from_forest_destructive(forest: [mutable @node]) -> @node {
        let i = 0u;
        let len = vec::len(forest);
        while len > 1u {
            i = 0u;
            while i < len - 1u {//Concat nodes 0 with 1, 2 with 3 etc.
                let left  = forest[i];
                let right = forest[i+1u];
                let left_len = char_len(left);
                let right_len= char_len(right);
                let left_height= height(left);
                let right_height=height(right);
                if left_len + right_len > hint_max_leaf_char_len {
                    if left_len <= hint_max_leaf_char_len {
                        left = flatten(left);
                        left_height = height(left);
                    }
                    if right_len <= hint_max_leaf_char_len {
                        right = flatten(right);
                        right_height = height(right);
                    }
                }
                if left_height >= hint_max_node_height {
                    left = of_substr_unsafer(@serialize_node(left),
                                             0u,byte_len(left),
                                             left_len);
                }
                if right_height >= hint_max_node_height {
                    right = of_substr_unsafer(@serialize_node(right),
                                             0u,byte_len(right),
                                             right_len);
                }
                forest[i/2u] = concat2(left, right);
                i += 2u;
            }
            if i == len - 1u {
                //And don't forget the last node if it is in even position
                forest[i/2u] = forest[i];
            }
            len = uint::div_ceil(len, 2u);
        }
        ret forest[0];
    }

    fn serialize_node(node: @node) -> str unsafe {
        let buf = vec::init_elt_mut(0u8, byte_len(node));
        let offset = 0u;//Current position in the buffer
        let it = leaf_iterator::start(node);
        while true {
            alt(leaf_iterator::next(it)) {
              option::none. { break; }
              option::some(x) {
                //TODO: Replace with memcpy or something similar
                let local_buf: [u8] = unsafe::reinterpret_cast(*x.content);
                let i = x.byte_offset;
                while i < x.byte_len {
                    buf[offset] = local_buf[i];
                    offset += 1u;
                    i      += 1u;
                }
                unsafe::leak(local_buf);
              }
            }
        }
        let str : str = unsafe::reinterpret_cast(buf);
        unsafe::leak(buf);//TODO: Check if this is correct
        ret str;
    }

    /*
    Function: flatten

    Replace a subtree by a single leaf with the same contents.

    Performance note: This function executes in linear time.
     */
    fn flatten(node: @node) -> @node unsafe {
        alt(*node) {
          leaf(_) { ret node }
          concat(x) {
            ret @leaf({
                byte_offset: 0u,
                byte_len:    x.byte_len,
                char_len:    x.char_len,
                content:     @serialize_node(node)
            })
          }
        }
    }

    /*
    Function: bal

    Balance a node.

    Algorithm:
    - if the node height is smaller than `hint_max_node_height`, do nothing
    - otherwise, gather all leaves as a forest, rebuild a balanced node,
         concatenating small leaves along the way

    Returns:
    - `option::none` if no transformation happened
    - `option::some(x)` otherwise, in which case `x` has the same contents
       as `node` bot lower height and/or fragmentation.
    */
    fn bal(node: @node) -> option::t<@node> {
        if height(node) < hint_max_node_height { ret option::none; }
        //1. Gather all leaves as a forest
        let forest = [mutable];
        let it = leaf_iterator::start(node);
        while true {
            alt (leaf_iterator::next(it)) {
              option::none.   { break; }
              option::some(x) { forest += [mutable @leaf(x)]; }
            }
        }
        //2. Rebuild tree from forest
        let root = @*tree_from_forest_destructive(forest);
        ret option::some(root);

    }

    /*
    Function: sub_bytes

    Compute the subnode of a node.

    Parameters:
    node        - A node
    byte_offset - A byte offset in `node`
    byte_len    - The number of bytes to return

    Performance notes:
    - this function performs no copying;
    - this function executes in a time proportional to the height of `node`.

    Safety notes:
    - this function fails if `byte_offset` or `byte_len` do not represent
    valid positions in `node`.
    */
    fn sub_bytes(node: @node, byte_offset: uint, byte_len: uint) -> @node {
        let node        = node;
        let byte_offset = byte_offset;
        while true {
            if byte_offset == 0u && byte_len == node::byte_len(node) {
                ret node;
            }
            alt(*node) {
              node::leaf(x) {
                let char_len =
                    str::char_len_range(*x.content, byte_offset, byte_len);
                ret @leaf({byte_offset: byte_offset,
                                byte_len:    byte_len,
                                char_len:    char_len,
                                content:     x.content});
              }
              node::concat(x) {
                let left_len: uint = node::byte_len(x.left);
                if byte_offset <= left_len {
                    if byte_offset + byte_len <= left_len {
                        //Case 1: Everything fits in x.left, tail-call
                        node = x.left;
                    } else {
                        //Case 2: A (non-empty, possibly full) suffix
                        //of x.left and a (non-empty, possibly full) prefix
                        //of x.right
                        let left_result  =
                            sub_bytes(x.left, byte_offset, left_len);
                        let right_result =
                            sub_bytes(x.right, 0u, left_len - byte_offset);
                        ret concat2(left_result, right_result);
                    }
                } else {
                    //Case 3: Everything fits in x.right
                    byte_offset -= left_len;
                    node = x.right;
                }
              }
            }
        }
        fail;//Note: unreachable
    }

    /*
    Function: sub_chars

    Compute the subnode of a node.

    Parameters:
    node        - A node
    char_offset - A char offset in `node`
    char_len    - The number of chars to return

    Performance notes:
    - this function performs no copying;
    - this function executes in a time proportional to the height of `node`.

    Safety notes:
    - this function fails if `char_offset` or `char_len` do not represent
    valid positions in `node`.
    */
    fn sub_chars(node: @node, char_offset: uint, char_len: uint) -> @node {
        let node        = node;
        let char_offset = char_offset;
        while true {
            alt(*node) {
              node::leaf(x) {
                if char_offset == 0u && char_len == x.char_len {
                    ret node;
                }
                let byte_offset =
                    str::byte_len_range(*x.content, 0u, char_offset);
                let byte_len    =
                    str::byte_len_range(*x.content, byte_offset, char_len);
                ret @leaf({byte_offset: byte_offset,
                           byte_len:    byte_len,
                           char_len:    char_len,
                           content:     x.content});
              }
              node::concat(x) {
                if char_offset == 0u && char_len == x.char_len {ret node;}
                let left_len : uint = node::char_len(x.left);
                if char_offset <= left_len {
                    if char_offset + char_len <= left_len {
                        //Case 1: Everything fits in x.left, tail call
                        node        = x.left;
                    } else {
                        //Case 2: A (non-empty, possibly full) suffix
                        //of x.left and a (non-empty, possibly full) prefix
                        //of x.right
                        let left_result  =
                            sub_chars(x.left, char_offset, left_len);
                        let right_result =
                            sub_chars(x.right, 0u, left_len - char_offset);
                        ret concat2(left_result, right_result);
                    }
                } else {
                    //Case 3: Everything fits in x.right, tail call
                    node = x.right;
                    char_offset -= left_len;
                }
              }
            }
        }
        fail;
    }

    fn concat2(left: @node, right: @node) -> @node {
        ret @concat({left    : left,
                     right   : right,
             char_len: char_len(left) + char_len(right),
                     byte_len: byte_len(left) + byte_len(right),
             height: math::max(height(left), height(right)) + 1u
                    })
    }

    fn height(node: @node) -> uint {
        alt(*node) {
          leaf(_)   { ret 0u; }
          concat(x) { ret x.height; }
        }
    }

    fn cmp(a: @node, b: @node) -> int {
        let ita = char_iterator::start(a);
        let itb = char_iterator::start(b);
        let result = 0;
        let pos = 0u;
        while result == 0 {
            alt((char_iterator::next(ita), char_iterator::next(itb))) {
              (option::none., option::none.) {
                break;
              }
              (option::some(chara), option::some(charb)) {
                result = char::cmp(chara, charb);
              }
              (option::some(_), _)         {
                result = 1;
              }
              (_, option::some(_))         {
                result = -1;
              }
            }
            pos += 1u;
        }
        ret result;
    }

    fn loop_chars(node: @node, it: block(char) -> bool) -> bool {
        ret loop_leaves(node, {|leaf|
            ret str::loop_chars_sub(*leaf.content,
                                    leaf.byte_offset,
                                    leaf.byte_len, it)
        })
    }

    /*
    Function: loop_leaves

    Loop through a node, leaf by leaf

    Parameters:

    rope - A node to traverse.
    it - A block to execute with each consecutive leaf of the node.
    Return `true` to continue, `false` to stop.

    Returns:

    `true` If execution proceeded correctly, `false` if it was interrupted,
    that is if `it` returned `false` at any point.
    */
    fn loop_leaves(node: @node, it: block(leaf) -> bool) -> bool{
        let current = node;
        while true {
            alt(*current) {
              leaf(x) {
                ret it(x);
              }
              concat(x) {
                if loop_leaves(x.left, it) { //non tail call
                    current = x.right;       //tail call
                } else {
                    ret false;
                }
              }
            }
        }
        fail;//unreachable
    }

    /*
    Function: char_at

    Parameters:
    pos - A position in the rope

    Returns: The character at position `pos`

    Safety notes: The function will fail if `pos`
    is not a valid position in the rope.

    Performance note: This function executes in a time
    proportional to the height of the rope + the (bounded)
    length of the largest leaf.
    */
    fn char_at(node: @node, pos: uint) -> char {
        let node    = node;
        let pos     = pos;
        while true {
            alt *node {
              leaf(x) {
                ret str::char_at(*x.content, pos);
              }
              concat({left, right, _}) {
                let left_len = char_len(left);
                node = if left_len > pos { left }
                       else { pos -= left_len; right };
              }
            }
        }
        fail;//unreachable
    }

    mod leaf_iterator {
        type t = {
            stack:            [mutable @node],
            mutable stackpos: int
        };

        fn empty() -> t {
            let stack : [mutable @node] = [mutable];
            ret {stack: stack, mutable stackpos: -1}
        }

        fn start(node: @node) -> t {
            let stack = vec::init_elt_mut(node, height(node)+1u);
            ret {
                stack:             stack,
                mutable stackpos:  0
            }
        }

        fn next(it: t) -> option::t<leaf> {
            if it.stackpos < 0 { ret option::none; }
            while true {
                let current = it.stack[it.stackpos];
                it.stackpos -= 1;
                alt(*current) {
                  concat(x) {
                    it.stackpos += 1;
                    it.stack[it.stackpos] = x.right;
                    it.stackpos += 1;
                    it.stack[it.stackpos] = x.left;
                  }
                  leaf(x) {
                    ret option::some(x);
                  }
                }
            }
            fail;//unreachable
        }
    }

    mod char_iterator {
        type t = {
            leaf_iterator: leaf_iterator::t,
            mutable leaf:  option::t<leaf>,
            mutable leaf_byte_pos: uint
        };

        fn start(node: @node) -> t {
            ret {
                leaf_iterator: leaf_iterator::start(node),
                mutable leaf:          option::none,
                mutable leaf_byte_pos: 0u
            }
        }

        fn empty() -> t {
            ret {
                leaf_iterator: leaf_iterator::empty(),
                mutable leaf:  option::none,
                mutable leaf_byte_pos: 0u
            }
        }

        fn next(it: t) -> option::t<char> {
            while true {
                alt(get_current_or_next_leaf(it)) {
                  option::none. { ret option::none; }
                  option::some(leaf) {
                    let next_char = get_next_char_in_leaf(it);
                    alt(next_char) {
                      option::none. {
                        cont;
                      }
                      option::some(_) {
                        ret next_char;
                      }
                    }
                  }
                }
            }
            fail;//unreachable
        }

        fn get_current_or_next_leaf(it: t) -> option::t<leaf> {
            alt(it.leaf) {
              option::some(_) { ret it.leaf }
              option::none. {
                let next = leaf_iterator::next(it.leaf_iterator);
                alt(next) {
                  option::none. { ret option::none }
                  option::some(leaf) {
                    it.leaf          = next;
                    it.leaf_byte_pos = 0u;
                    ret next;
                  }
                }
              }
            }
        }

        fn get_next_char_in_leaf(it: t) -> option::t<char> {
            alt(it.leaf) {
              option::none. { ret option::none }
              option::some(leaf) {
                if it.leaf_byte_pos >= leaf.byte_len {
                    //We are actually past the end of the leaf
                    it.leaf = option::none;
                    ret option::none
                } else {
                    let {ch, next} =
                        str::char_range_at(*leaf.content,
                                     it.leaf_byte_pos + leaf.byte_offset);
                    it.leaf_byte_pos = next - leaf.byte_offset;
                    ret option::some(ch)
                }
              }
            }
        }
    }
}