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Merge pull request #6259 from dec05eba/master

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Use boyer-moore-horspool algorithm for indexOfPos and lastIndexOf unless the haystack or needle is very small
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Andrew Kelley 2020-10-17 21:38:50 -04:00 committed by GitHub
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1 changed files with 85 additions and 8 deletions
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lib/std/mem.zig
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@ -853,10 +853,7 @@ pub fn indexOf(comptime T: type, haystack: []const T, needle: []const T) ?usize
/// Find the index in a slice of a sub-slice, searching from the end backwards.
/// To start looking at a different index, slice the haystack first.
/// TODO is there even a better algorithm for this?
pub fn lastIndexOf(comptime T: type, haystack: []const T, needle: []const T) ?usize {
if (needle.len > haystack.len) return null;
fn lastIndexOfLinear(comptime T: type, haystack: []const T, needle: []const T) ?usize {
var i: usize = haystack.len - needle.len;
while (true) : (i -= 1) {
if (mem.eql(T, haystack[i .. i + needle.len], needle)) return i;
@ -864,10 +861,7 @@ pub fn lastIndexOf(comptime T: type, haystack: []const T, needle: []const T) ?us
}
}
// TODO boyer-moore algorithm
pub fn indexOfPos(comptime T: type, haystack: []const T, start_index: usize, needle: []const T) ?usize {
if (needle.len > haystack.len) return null;
fn indexOfPosLinear(comptime T: type, haystack: []const T, start_index: usize, needle: []const T) ?usize {
var i: usize = start_index;
const end = haystack.len - needle.len;
while (i <= end) : (i += 1) {
@ -876,7 +870,90 @@ pub fn indexOfPos(comptime T: type, haystack: []const T, start_index: usize, nee
return null;
}
fn boyerMooreHorspoolPreprocessReverse(pattern: []const u8, table: *[256]usize) void {
for (table) |*c| {
c.* = pattern.len;
}
var i: usize = pattern.len - 1;
// The first item is intentionally ignored and the skip size will be pattern.len.
// This is the standard way boyer-moore-horspool is implemented.
while (i > 0) : (i -= 1) {
table[pattern[i]] = i;
}
}
fn boyerMooreHorspoolPreprocess(pattern: []const u8, table: *[256]usize) void {
for (table) |*c| {
c.* = pattern.len;
}
var i: usize = 0;
// The last item is intentionally ignored and the skip size will be pattern.len.
// This is the standard way boyer-moore-horspool is implemented.
while (i < pattern.len - 1) : (i += 1) {
table[pattern[i]] = pattern.len - 1 - i;
}
}
/// Find the index in a slice of a sub-slice, searching from the end backwards.
/// To start looking at a different index, slice the haystack first.
// Reverse boyer-moore-horspool algorithm
pub fn lastIndexOf(comptime T: type, haystack: []const T, needle: []const T) ?usize {
if (needle.len > haystack.len) return null;
if (needle.len == 0) return haystack.len;
if (!meta.trait.hasUniqueRepresentation(T) or haystack.len < 32 or needle.len <= 2)
return lastIndexOfLinear(T, haystack, needle);
const haystack_bytes = sliceAsBytes(haystack);
const needle_bytes = sliceAsBytes(needle);
var skip_table: [256]usize = undefined;
boyerMooreHorspoolPreprocessReverse(needle_bytes, skip_table[0..]);
var i: usize = haystack_bytes.len - needle_bytes.len;
while (true) {
if (mem.eql(u8, haystack_bytes[i .. i + needle_bytes.len], needle_bytes)) return i;
const skip = skip_table[haystack_bytes[i]];
if (skip > i) break;
i -= skip;
}
return null;
}
// Boyer-moore-horspool algorithm
pub fn indexOfPos(comptime T: type, haystack: []const T, start_index: usize, needle: []const T) ?usize {
if (needle.len > haystack.len) return null;
if (needle.len == 0) return 0;
if (!meta.trait.hasUniqueRepresentation(T) or haystack.len < 52 or needle.len <= 4)
return indexOfPosLinear(T, haystack, start_index, needle);
const haystack_bytes = sliceAsBytes(haystack);
const needle_bytes = sliceAsBytes(needle);
var skip_table: [256]usize = undefined;
boyerMooreHorspoolPreprocess(needle_bytes, skip_table[0..]);
var i: usize = start_index * @sizeOf(T);
while (i <= haystack_bytes.len - needle_bytes.len) {
if (mem.eql(u8, haystack_bytes[i .. i + needle_bytes.len], needle_bytes)) return i;
i += skip_table[haystack_bytes[i + needle_bytes.len - 1]];
}
return null;
}
test "mem.indexOf" {
testing.expect(indexOf(u8, "one two three four five six seven eight nine ten", "three four").? == 8);
testing.expect(lastIndexOf(u8, "one two three four five six seven eight nine ten", "three four").? == 8);
testing.expect(indexOf(u8, "one two three four five six seven eight nine ten", "two two") == null);
testing.expect(lastIndexOf(u8, "one two three four five six seven eight nine ten", "two two") == null);
testing.expect(indexOf(u8, "one two three four five six seven eight nine ten", "").? == 0);
testing.expect(lastIndexOf(u8, "one two three four five six seven eight nine ten", "").? == 48);
testing.expect(indexOf(u8, "one two three four", "four").? == 14);
testing.expect(lastIndexOf(u8, "one two three two four", "two").? == 14);
testing.expect(indexOf(u8, "one two three four", "gour") == null);