//! Implements a map from integer indices to data. //! Rather than storing data for every index, internally, this maps entire ranges to the data. //! To this end, the APIs all work on ranges, not on individual integers. Ranges are split as //! necessary (e.g., when [0,5) is first associated with X, and then [1,2) is mutated). //! Users must not depend on whether a range is coalesced or not, even though this is observable //! via the iteration APIs. use std::ops; use rustc_target::abi::Size; #[derive(Clone, Debug)] struct Elem { /// The range covered by this element; never empty. range: ops::Range, /// The data stored for this element. data: T, } #[derive(Clone, Debug)] pub struct RangeMap { v: Vec>, } impl RangeMap { /// Creates a new `RangeMap` for the given size, and with the given initial value used for /// the entire range. #[inline(always)] pub fn new(size: Size, init: T) -> RangeMap { let size = size.bytes(); let mut map = RangeMap { v: Vec::new() }; if size > 0 { map.v.push(Elem { range: 0..size, data: init }); } map } /// Finds the index containing the given offset. fn find_offset(&self, offset: u64) -> usize { // We do a binary search. let mut left = 0usize; // inclusive let mut right = self.v.len(); // exclusive loop { debug_assert!(left < right, "find_offset: offset {} is out-of-bounds", offset); let candidate = left.checked_add(right).unwrap() / 2; let elem = &self.v[candidate]; if offset < elem.range.start { // We are too far right (offset is further left). debug_assert!(candidate < right); // we are making progress right = candidate; } else if offset >= elem.range.end { // We are too far left (offset is further right). debug_assert!(candidate >= left); // we are making progress left = candidate + 1; } else { // This is it! return candidate; } } } /// Provides read-only iteration over everything in the given range. This does /// *not* split items if they overlap with the edges. Do not use this to mutate /// through interior mutability. /// /// The iterator also provides the offset of the given element. pub fn iter(&self, offset: Size, len: Size) -> impl Iterator { let offset = offset.bytes(); let len = len.bytes(); // Compute a slice starting with the elements we care about. let slice: &[Elem] = if len == 0 { // We just need any empty iterator. We don't even want to // yield the element that surrounds this position. &[] } else { let first_idx = self.find_offset(offset); &self.v[first_idx..] }; // The first offset that is not included any more. let end = offset + len; slice .iter() .take_while(move |elem| elem.range.start < end) .map(|elem| (Size::from_bytes(elem.range.start), &elem.data)) } pub fn iter_mut_all(&mut self) -> impl Iterator { self.v.iter_mut().map(|elem| &mut elem.data) } // Splits the element situated at the given `index`, such that the 2nd one starts at offset // `split_offset`. Do nothing if the element already starts there. // Returns whether a split was necessary. fn split_index(&mut self, index: usize, split_offset: u64) -> bool where T: Clone, { let elem = &mut self.v[index]; if split_offset == elem.range.start || split_offset == elem.range.end { // Nothing to do. return false; } debug_assert!( elem.range.contains(&split_offset), "the `split_offset` is not in the element to be split" ); // Now we really have to split. Reduce length of first element. let second_range = split_offset..elem.range.end; elem.range.end = split_offset; // Copy the data, and insert second element. let second = Elem { range: second_range, data: elem.data.clone() }; self.v.insert(index + 1, second); return true; } /// Provides mutable iteration over everything in the given range. As a side-effect, /// this will split entries in the map that are only partially hit by the given range, /// to make sure that when they are mutated, the effect is constrained to the given range. /// Moreover, this will opportunistically merge neighbouring equal blocks. /// /// The iterator also provides the offset of the given element. pub fn iter_mut(&mut self, offset: Size, len: Size) -> impl Iterator where T: Clone + PartialEq, { let offset = offset.bytes(); let len = len.bytes(); // Compute a slice containing exactly the elements we care about let slice: &mut [Elem] = if len == 0 { // We just need any empty iterator. We don't even want to // yield the element that surrounds this position, nor do // any splitting. &mut [] } else { // Make sure we got a clear beginning let mut first_idx = self.find_offset(offset); if self.split_index(first_idx, offset) { // The newly created 2nd element is ours first_idx += 1; } // No more mutation. let first_idx = first_idx; // Find our end. Linear scan, but that's ok because the iteration // is doing the same linear scan anyway -- no increase in complexity. // We combine this scan with a scan for duplicates that we can merge, to reduce // the number of elements. // We stop searching after the first "block" of size 1, to avoid spending excessive // amounts of time on the merging. let mut equal_since_idx = first_idx; // Once we see too many non-mergeable blocks, we stop. // The initial value is chosen via... magic. Benchmarking and magic. let mut successful_merge_count = 3usize; // When the loop is done, this is the first excluded element. let mut end_idx = first_idx; loop { // Compute if `end` is the last element we need to look at. let done = self.v[end_idx].range.end >= offset + len; // We definitely need to include `end`, so move the index. end_idx += 1; debug_assert!( done || end_idx < self.v.len(), "iter_mut: end-offset {} is out-of-bounds", offset + len ); // see if we want to merge everything in `equal_since..end` (exclusive at the end!) if successful_merge_count > 0 { if done || self.v[end_idx].data != self.v[equal_since_idx].data { // Everything in `equal_since..end` was equal. Make them just one element covering // the entire range. let removed_elems = end_idx - equal_since_idx - 1; // number of elements that we would remove if removed_elems > 0 { // Adjust the range of the first element to cover all of them. let equal_until = self.v[end_idx - 1].range.end; // end of range of last of the equal elements self.v[equal_since_idx].range.end = equal_until; // Delete the rest of them. self.v.splice(equal_since_idx + 1..end_idx, std::iter::empty()); // Adjust `end_idx` because we made the list shorter. end_idx -= removed_elems; // Adjust the count for the cutoff. successful_merge_count += removed_elems; } else { // Adjust the count for the cutoff. successful_merge_count -= 1; } // Go on scanning for the next block starting here. equal_since_idx = end_idx; } } // Leave loop if this is the last element. if done { break; } } // Move to last included instead of first excluded index. let end_idx = end_idx - 1; // We need to split the end as well. Even if this performs a // split, we don't have to adjust our index as we only care about // the first part of the split. self.split_index(end_idx, offset + len); // Now we yield the slice. `end` is inclusive. &mut self.v[first_idx..=end_idx] }; slice.iter_mut().map(|elem| (Size::from_bytes(elem.range.start), &mut elem.data)) } } #[cfg(test)] mod tests { use super::*; /// Query the map at every offset in the range and collect the results. fn to_vec(map: &RangeMap, offset: u64, len: u64) -> Vec { (offset..offset + len) .into_iter() .map(|i| { map.iter(Size::from_bytes(i), Size::from_bytes(1)).next().map(|(_, &t)| t).unwrap() }) .collect() } #[test] fn basic_insert() { let mut map = RangeMap::::new(Size::from_bytes(20), -1); // Insert. for (_, x) in map.iter_mut(Size::from_bytes(10), Size::from_bytes(1)) { *x = 42; } // Check. assert_eq!(to_vec(&map, 10, 1), vec![42]); assert_eq!(map.v.len(), 3); // Insert with size 0. for (_, x) in map.iter_mut(Size::from_bytes(10), Size::from_bytes(0)) { *x = 19; } for (_, x) in map.iter_mut(Size::from_bytes(11), Size::from_bytes(0)) { *x = 19; } assert_eq!(to_vec(&map, 10, 2), vec![42, -1]); assert_eq!(map.v.len(), 3); } #[test] fn gaps() { let mut map = RangeMap::::new(Size::from_bytes(20), -1); for (_, x) in map.iter_mut(Size::from_bytes(11), Size::from_bytes(1)) { *x = 42; } for (_, x) in map.iter_mut(Size::from_bytes(15), Size::from_bytes(1)) { *x = 43; } assert_eq!(map.v.len(), 5); assert_eq!(to_vec(&map, 10, 10), vec![-1, 42, -1, -1, -1, 43, -1, -1, -1, -1]); for (_, x) in map.iter_mut(Size::from_bytes(10), Size::from_bytes(10)) { if *x < 42 { *x = 23; } } assert_eq!(map.v.len(), 6); assert_eq!(to_vec(&map, 10, 10), vec![23, 42, 23, 23, 23, 43, 23, 23, 23, 23]); assert_eq!(to_vec(&map, 13, 5), vec![23, 23, 43, 23, 23]); for (_, x) in map.iter_mut(Size::from_bytes(15), Size::from_bytes(5)) { *x = 19; } assert_eq!(map.v.len(), 6); assert_eq!(to_vec(&map, 10, 10), vec![23, 42, 23, 23, 23, 19, 19, 19, 19, 19]); // Should be seeing two blocks with 19. assert_eq!( map.iter(Size::from_bytes(15), Size::from_bytes(2)) .map(|(_, &t)| t) .collect::>(), vec![19, 19] ); // A NOP `iter_mut` should trigger merging. for _ in map.iter_mut(Size::from_bytes(15), Size::from_bytes(5)) {} assert_eq!(map.v.len(), 5); assert_eq!(to_vec(&map, 10, 10), vec![23, 42, 23, 23, 23, 19, 19, 19, 19, 19]); } }