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std.fmt: Fix incorrect behavior with large floating point integers.

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I consider this an interim workaround/hack until #1299 is finished.

There is a bug in the original C implementation of the errol3 (and errol4)
algorithm that can result in undefined behavior or an obviously incorrect
result (leading ':' in the output)

This change checks for those two problems and uses a slower fallback
path if they occur. I can't guarantee that this will always produce
the correct result, but since the workaround is only used if the original
algorithm is guaranteed to fail, it should never turn a previously-correct
result into an incorrect one.

Fixes #11283
This commit is contained in:
Evan Haas 2022-04-03 08:47:17 -07:00 committed by Andrew Kelley
parent 364e53f3bf
commit b4bf3bdf7e
2 changed files with 8 additions and 1 deletions
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2
lib/std/fmt.zig
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@ -2299,6 +2299,8 @@ test "float.decimal" {
try expectFmt("f64: 0.00000", "f64: {d:.5}", .{@as(f64, 1.40130e-45)});
try expectFmt("f64: 0.00000", "f64: {d:.5}", .{@as(f64, 9.999960e-40)});
try expectFmt("f64: 10000000000000.00", "f64: {d:.2}", .{@as(f64, 9999999999999.999)});
try expectFmt("f64: 10000000000000000000000000000000000000", "f64: {d}", .{@as(f64, 1e37)});
try expectFmt("f64: 100000000000000000000000000000000000000", "f64: {d}", .{@as(f64, 1e38)});
}
test "float.libc.sanity" {

7
lib/std/fmt/errol.zig
View File

@ -106,7 +106,10 @@ fn errol3u(val: f64, buffer: []u8) FloatDecimal {
} else if (val >= 16.0 and val < 9.007199254740992e15) {
return errolFixed(val, buffer);
}
return errolSlow(val, buffer);
}
fn errolSlow(val: f64, buffer: []u8) FloatDecimal {
// normalize the midpoint
const e = math.frexp(val).exponent;
@ -336,7 +339,9 @@ fn errolInt(val: f64, buffer: []u8) FloatDecimal {
var buf_index = u64toa(m64, buffer) - 1;
if (mi != 0) {
buffer[buf_index - 1] += @boolToInt(buffer[buf_index] >= '5');
const round_up = buffer[buf_index] >= '5';
if (buf_index == 0 or (round_up and buffer[buf_index - 1] == '9')) return errolSlow(val, buffer);
buffer[buf_index - 1] += @boolToInt(round_up);
} else {
buf_index += 1;
}