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use in-place swap for List.reverse

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This commit is contained in:
Folkert 2021-04-04 21:42:02 +02:00
parent ba0408a587
commit 4c9accb2ba
1 changed files with 50 additions and 51 deletions
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101
compiler/builtins/bitcode/src/list.zig
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@ -124,33 +124,18 @@ pub fn listReverse(list: RocList, alignment: usize, element_width: usize) callco
const size = list.len();
var i: usize = 0;
var end: usize = size - 1;
const end: usize = size - 1;
if (list.isUnique()) {
const temp: [*]u8 = @ptrCast([*]u8, std.heap.c_allocator.alloc(u8, element_width) catch unreachable);
// Working from the front and back so
// we only need to go ~(n / 2) iterations.
// If the length is an odd number the middle
// element stays in the same place anyways.
while (i < (end - i)) : (i += 1) {
const last_position = end - i;
const last_element = source_ptr + (last_position * element_width);
const first_element = source_ptr + (i * element_width);
// Store Last Element in temp
@memcpy(temp, last_element, element_width);
// Swap Last Element with First Element
@memcpy(last_element, first_element, element_width);
// Swap First Element with temp
@memcpy(first_element, temp, element_width);
swapElements(source_ptr, element_width, i, end - i);
}
std.heap.c_allocator.free(temp[0..element_width]);
return list;
} else {
const output = RocList.allocate(std.heap.c_allocator, alignment, size, element_width);
@ -743,38 +728,6 @@ fn listRangeHelp(allocator: *Allocator, comptime T: type, low: T, high: T) RocLi
}
}
inline fn swapHelp(width: usize, temporary: [*]u8, ptr1: [*]u8, ptr2: [*]u8) void {
@memcpy(temporary, ptr1, width);
@memcpy(ptr1, ptr2, width);
@memcpy(ptr2, temporary, width);
}
fn swap(source_ptr: [*]u8, element_width_initial: usize, index_1: usize, index_2: usize) void {
const threshold: comptime usize = 64;
var element_width = element_width_initial;
var buffer_actual: [threshold]u8 = undefined;
var buffer: [*]u8 = buffer_actual[0..];
var element_at_i = source_ptr + (index_1 * element_width);
var element_at_j = source_ptr + (index_2 * element_width);
while (true) {
if (element_width < threshold) {
swapHelp(element_width, buffer, element_at_i, element_at_j);
return;
} else {
swapHelp(threshold, buffer, element_at_i, element_at_j);
element_at_i += threshold;
element_at_j += threshold;
element_width -= threshold;
}
}
}
fn partition(source_ptr: [*]u8, transform: Opaque, wrapper: CompareFn, element_width: usize, low: isize, high: isize) isize {
const pivot = source_ptr + (@intCast(usize, high) * element_width);
var i = (low - 1); // Index of smaller element and indicates the right position of pivot found so far
@ -790,12 +743,12 @@ fn partition(source_ptr: [*]u8, transform: Opaque, wrapper: CompareFn, element_w
utils.Ordering.LT => {
// the current element is smaller than the pivot; swap it
i += 1;
swap(source_ptr, element_width, @intCast(usize, i), @intCast(usize, j));
swapElements(source_ptr, element_width, @intCast(usize, i), @intCast(usize, j));
},
utils.Ordering.EQ, utils.Ordering.GT => {},
}
}
swap(source_ptr, element_width, @intCast(usize, i + 1), @intCast(usize, high));
swapElements(source_ptr, element_width, @intCast(usize, i + 1), @intCast(usize, high));
return (i + 1);
}
@ -820,3 +773,49 @@ pub fn listSortWith(input: RocList, transform: Opaque, wrapper: CompareFn, align
return list;
}
// SWAP ELEMENTS
inline fn swapHelp(width: usize, temporary: [*]u8, ptr1: [*]u8, ptr2: [*]u8) void {
@memcpy(temporary, ptr1, width);
@memcpy(ptr1, ptr2, width);
@memcpy(ptr2, temporary, width);
}
fn swap(width_initial: usize, p1: [*]u8, p2: [*]u8) void {
const threshold: comptime usize = 64;
var width = width_initial;
var ptr1 = p1;
var ptr2 = p2;
var buffer_actual: [threshold]u8 = undefined;
var buffer: [*]u8 = buffer_actual[0..];
while (true) {
if (width < threshold) {
swapHelp(width, buffer, ptr1, ptr2);
return;
} else {
swapHelp(threshold, buffer, ptr1, ptr2);
ptr1 += threshold;
ptr2 += threshold;
width -= threshold;
}
}
}
fn swapElements(source_ptr: [*]u8, element_width: usize, index_1: usize, index_2: usize) void {
const threshold: comptime usize = 64;
var buffer_actual: [threshold]u8 = undefined;
var buffer: [*]u8 = buffer_actual[0..];
var element_at_i = source_ptr + (index_1 * element_width);
var element_at_j = source_ptr + (index_2 * element_width);
return swap(element_width, element_at_i, element_at_j);
}