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std.unicode: Add ASCII fast path to UTF-16 -> UTF-8 conversion functions

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This commit is contained in:
Ryan Liptak 2023-10-30 17:12:30 -07:00
parent 03117c5290
commit 13c8ec9db0
1 changed files with 81 additions and 5 deletions
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86
lib/std/unicode.zig
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@ -757,8 +757,34 @@ pub fn utf16leToUtf8Alloc(allocator: mem.Allocator, utf16le: []const u16) ![]u8
// optimistically guess that it will all be ascii.
var result = try std.ArrayList(u8).initCapacity(allocator, utf16le.len);
errdefer result.deinit();
var out_index: usize = 0;
var it = Utf16LeIterator.init(utf16le);
var remaining = utf16le;
if (builtin.zig_backend != .stage2_x86_64) {
const chunk_len = std.simd.suggestVectorSize(u16) orelse 1;
const Chunk = @Vector(chunk_len, u16);
// Fast path. Check for and encode ASCII characters at the start of the input.
while (remaining.len >= chunk_len) {
const chunk: Chunk = remaining[0..chunk_len].*;
const mask: Chunk = @splat(std.mem.nativeToLittle(u16, 0x7F));
if (@reduce(.Or, chunk | mask != mask)) {
// found a non ASCII code unit
break;
}
const chunk_byte_len = chunk_len * 2;
const chunk_bytes: @Vector(chunk_byte_len, u8) = (std.mem.sliceAsBytes(remaining)[0..chunk_byte_len]).*;
const deinterlaced_bytes = std.simd.deinterlace(2, chunk_bytes);
const ascii_bytes: [chunk_len]u8 = deinterlaced_bytes[0];
// We allocated enough space to encode every UTF-16 code unit
// as ASCII, so if the entire string is ASCII then we are
// guaranteed to have enough space allocated
result.appendSliceAssumeCapacity(&ascii_bytes);
remaining = remaining[chunk_len..];
}
}
var out_index: usize = result.items.len;
var it = Utf16LeIterator.init(remaining);
while (try it.nextCodepoint()) |codepoint| {
const utf8_len = utf8CodepointSequenceLength(codepoint) catch unreachable;
try result.resize(result.items.len + utf8_len);
@ -774,8 +800,34 @@ pub fn utf16leToUtf8AllocZ(allocator: mem.Allocator, utf16le: []const u16) ![:0]
// optimistically guess that it will all be ascii (and allocate space for the null terminator)
var result = try std.ArrayList(u8).initCapacity(allocator, utf16le.len + 1);
errdefer result.deinit();
var out_index: usize = 0;
var it = Utf16LeIterator.init(utf16le);
var remaining = utf16le;
if (builtin.zig_backend != .stage2_x86_64) {
const chunk_len = std.simd.suggestVectorSize(u16) orelse 1;
const Chunk = @Vector(chunk_len, u16);
// Fast path. Check for and encode ASCII characters at the start of the input.
while (remaining.len >= chunk_len) {
const chunk: Chunk = remaining[0..chunk_len].*;
const mask: Chunk = @splat(std.mem.nativeToLittle(u16, 0x7F));
if (@reduce(.Or, chunk | mask != mask)) {
// found a non ASCII code unit
break;
}
const chunk_byte_len = chunk_len * 2;
const chunk_bytes: @Vector(chunk_byte_len, u8) = (std.mem.sliceAsBytes(remaining)[0..chunk_byte_len]).*;
const deinterlaced_bytes = std.simd.deinterlace(2, chunk_bytes);
const ascii_bytes: [chunk_len]u8 = deinterlaced_bytes[0];
// We allocated enough space to encode every UTF-16 code unit
// as ASCII, so if the entire string is ASCII then we are
// guaranteed to have enough space allocated
result.appendSliceAssumeCapacity(&ascii_bytes);
remaining = remaining[chunk_len..];
}
}
var out_index = result.items.len;
var it = Utf16LeIterator.init(remaining);
while (try it.nextCodepoint()) |codepoint| {
const utf8_len = utf8CodepointSequenceLength(codepoint) catch unreachable;
try result.resize(result.items.len + utf8_len);
@ -789,7 +841,31 @@ pub fn utf16leToUtf8AllocZ(allocator: mem.Allocator, utf16le: []const u16) ![:0]
/// Returns end byte index into utf8.
pub fn utf16leToUtf8(utf8: []u8, utf16le: []const u16) !usize {
var end_index: usize = 0;
var it = Utf16LeIterator.init(utf16le);
var remaining = utf16le;
if (builtin.zig_backend != .stage2_x86_64) {
const chunk_len = std.simd.suggestVectorSize(u16) orelse 1;
const Chunk = @Vector(chunk_len, u16);
// Fast path. Check for and encode ASCII characters at the start of the input.
while (remaining.len >= chunk_len) {
const chunk: Chunk = remaining[0..chunk_len].*;
const mask: Chunk = @splat(std.mem.nativeToLittle(u16, 0x7F));
if (@reduce(.Or, chunk | mask != mask)) {
// found a non ASCII code unit
break;
}
const chunk_byte_len = chunk_len * 2;
const chunk_bytes: @Vector(chunk_byte_len, u8) = (std.mem.sliceAsBytes(remaining)[0..chunk_byte_len]).*;
const deinterlaced_bytes = std.simd.deinterlace(2, chunk_bytes);
const ascii_bytes: [chunk_len]u8 = deinterlaced_bytes[0];
@memcpy(utf8[end_index .. end_index + chunk_len], &ascii_bytes);
end_index += chunk_len;
remaining = remaining[chunk_len..];
}
}
var it = Utf16LeIterator.init(remaining);
while (try it.nextCodepoint()) |codepoint| {
end_index += try utf8Encode(codepoint, utf8[end_index..]);
}