style-guide: Rework version-sorting algorithm

Treat numeric chunks with equal value but differing numbers of leading
zeroes as equal, unless we get to the end of the entire string in which
case we use "more leading zeroes in the earliest differing chunk" as a
tiebreaker.

Treat `_` as a word separator, sorting it before anything other than
space.

Give more examples.

src/doc/style-guide/src/README.md

@@ -109,32 +109,79 @@ lexicographical.)
 
 For the purposes of the Rust style, to compare two strings for version-sorting:
 
-- Compare the strings by (Unicode) character lexicographically, finding the
-  index of the first differing character. (If the two strings do not have the
-  same length, this may be the end of the shorter string.)
-- For both strings, determine the longest sequence of ASCII digits that either
-  contains or ends at that index. (If either string doesn't have such a
-  sequence of ASCII digits, fall back to comparing the strings
-  lexicographically.)
-- Compare the numeric values of the number specified by the sequence of digits.
-  (Note that an implementation of this algorithm can easily check this without
-  accumulating copies of the digits or converting to a number: after skipping
-  leading zeroes, longer sequences of digits are larger numbers, and
-  equal-length sequences of digits can be sorted lexicographically.)
-- If the numbers have the same numeric value, the one with more leading zeroes
-  comes first.
+- Process both strings from beginning to end as two sequences of maximal-length
+  chunks, where each chunk consists either of a sequence of characters other
+  than ASCII digits, or a sequence of ASCII digits (a numeric chunk), and
+  compare corresponding chunks from the strings.
+- To compare two numeric chunks, compare them by numeric value, ignoring
+  leading zeroes. If the two chunks have equal numeric value, but different
+  numbers of leading digits, and this is the first time this has happened for
+  these strings, treat the chunks as equal (moving on to the next chunk) but
+  remember which string had more leading zeroes.
+- To compare two chunks if both are not numeric, compare them by Unicode
+  character lexicographically, except that `_` (underscore) sorts immediately
+  after ` ` (space) but before any other character. (This treats underscore as
+  a word separator, as commonly used in identifiers.)
+  - If the use of version sorting specifies further modifiers, such as sorting
+    non-lowercase before lowercase, apply those modifiers to the lexicographic
+    sort in this step.
+- If the comparison reaches the end of the string and considers each pair of
+  chunks equal:
+  - If one of the numeric comparisons noted the earliest point at which one
+    string had more leading zeroes than the other, sort the string with more
+    leading zeroes first.
+  - Otherwise, the strings are equal.
 
-Note that there exist various algorithms called "version sorting", which differ
-most commonly in their handling of numbers with leading zeroes. This algorithm
+Note that there exist various algorithms called "version sorting", which
+generally try to solve the same problem, but which differ in various ways (such
+as in their handling of numbers with leading zeroes). This algorithm
 does not purport to precisely match the behavior of any particular other
 algorithm, only to produce a simple and satisfying result for Rust formatting.
-(In particular, this algorithm aims to produce a satisfying result for a set of
+In particular, this algorithm aims to produce a satisfying result for a set of
 symbols that have the same number of leading zeroes, and an acceptable and
 easily understandable result for a set of symbols that has varying numbers of
-leading zeroes.)
+leading zeroes.
 
-As an example, version-sorting will sort the following symbols in the order
-given: `x000`, `x00`, `x0`, `x01`, `x1`, `x09`, `x9`, `x010`, `x10`.
+As an example, version-sorting will sort the following strings in the order
+given:
+- `_ZYWX`
+- `u_zzz`
+- `u8`
+- `u16`
+- `u32`
+- `u64`
+- `u128`
+- `u256`
+- `ua`
+- `usize`
+- `uz`
+- `v000`
+- `v00`
+- `v0`
+- `v0s`
+- `v00t`
+- `v0u`
+- `v001`
+- `v01`
+- `v1`
+- `v009`
+- `v09`
+- `v9`
+- `v010`
+- `v10`
+- `w005s09t`
+- `w5s009t`
+- `x64`
+- `x86`
+- `x86_32`
+- `x86_64`
+- `x86_128`
+- `x87`
+- `Z_YWX`
+- `ZY_WX`
+- `ZYW_X`
+- `ZYWX`
+- `ZYWX_`
 
 ### [Module-level items](items.md)