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fix #17142, wrong comptime log_int computation

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Guillaume Wenzek 2024-01-12 17:28:56 +01:00 committed by Andrew Kelley
parent d55d1e32b6
commit c5d359e4cd
3 changed files with 19 additions and 6 deletions
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2
lib/std/math.zig
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@ -672,6 +672,7 @@ test "rotl" {
/// - 1. Suitable for 0-based bit indices of T.
pub fn Log2Int(comptime T: type) type {
// comptime ceil log2
if (T == comptime_int) return comptime_int;
comptime var count = 0;
comptime var s = @typeInfo(T).Int.bits - 1;
inline while (s != 0) : (s >>= 1) {
@ -684,6 +685,7 @@ pub fn Log2Int(comptime T: type) type {
/// Returns an unsigned int type that can hold the number of bits in T.
pub fn Log2IntCeil(comptime T: type) type {
// comptime ceil log2
if (T == comptime_int) return comptime_int;
comptime var count = 0;
comptime var s = @typeInfo(T).Int.bits;
inline while (s != 0) : (s >>= 1) {

5
lib/std/math/log.zig
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@ -24,11 +24,8 @@ pub fn log(comptime T: type, base: T, x: T) T {
return @as(comptime_float, @log(@as(f64, x)) / @log(float_base));
},
// TODO: implement integer log without using float math.
// The present implementation is incorrect, for example
// `log(comptime_int, 9, 59049)` should return `5` and not `4`.
.ComptimeInt => {
return @as(comptime_int, @floor(@log(@as(f64, x)) / @log(float_base)));
return @as(comptime_int, math.log_int(comptime_int, base, x));
},
.Int => |IntType| switch (IntType.signedness) {

18
lib/std/math/log_int.zig
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@ -7,10 +7,15 @@ const Log2Int = math.Log2Int;
/// Returns the logarithm of `x` for the provided `base`, rounding down to the nearest integer.
/// Asserts that `base > 1` and `x > 0`.
pub fn log_int(comptime T: type, base: T, x: T) Log2Int(T) {
if (@typeInfo(T) != .Int or @typeInfo(T).Int.signedness != .unsigned)
@compileError("log_int requires an unsigned integer, found " ++ @typeName(T));
const valid = switch (@typeInfo(T)) {
.ComptimeInt => true,
.Int => |IntType| IntType.signedness == .unsigned,
else => false,
};
if (!valid) @compileError("log_int requires an unsigned integer, found " ++ @typeName(T));
assert(base > 1 and x > 0);
if (base == 2) return math.log2_int(T, x);
// Let's denote by [y] the integer part of y.
@ -112,3 +117,12 @@ test "math.log_int vs math.log10" {
}
}
}
test "math.log_int at comptime" {
const x = 59049; // 9 ** 5;
comptime {
if (math.log_int(comptime_int, 9, x) != 5) {
@compileError("log(9, 59049) should be 5");
}
}
}