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improve bit shift API in std.math

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* `shl` moved to `shlExact`
 * added `shl` and `shr` which are truncating like `<<` and `>>`.

closes #403
This commit is contained in:
Andrew Kelley 2017-09-30 21:09:58 -04:00
parent c6295fe9ab
commit 844e05f619
1 changed files with 50 additions and 2 deletions
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52
std/math/index.zig
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@ -219,11 +219,59 @@ pub fn negate(x: var) -> %@typeOf(x) {
}
error Overflow;
pub fn shl(comptime T: type, a: T, shift_amt: Log2Int(T)) -> %T {
pub fn shlExact(comptime T: type, a: T, shift_amt: Log2Int(T)) -> %T {
var answer: T = undefined;
if (@shlWithOverflow(T, a, shift_amt, &answer)) error.Overflow else answer
}
/// Shifts left. Overflowed bits are truncated.
/// A negative shift amount results in a right shift.
pub fn shl(comptime T: type, a: T, shift_amt: var) -> T {
const abs_shift_amt = absCast(shift_amt);
const casted_shift_amt = if (abs_shift_amt >= T.bit_count) return 0 else Log2Int(T)(abs_shift_amt);
if (@typeOf(shift_amt).is_signed) {
if (shift_amt >= 0) {
return a << casted_shift_amt;
} else {
return a >> casted_shift_amt;
}
}
return a << casted_shift_amt;
}
test "math.shl" {
assert(shl(u8, 0b11111111, usize(3)) == 0b11111000);
assert(shl(u8, 0b11111111, usize(8)) == 0);
assert(shl(u8, 0b11111111, usize(9)) == 0);
assert(shl(u8, 0b11111111, isize(-2)) == 0b00111111);
}
/// Shifts right. Overflowed bits are truncated.
/// A negative shift amount results in a lefft shift.
pub fn shr(comptime T: type, a: T, shift_amt: var) -> T {
const abs_shift_amt = absCast(shift_amt);
const casted_shift_amt = if (abs_shift_amt >= T.bit_count) return 0 else Log2Int(T)(abs_shift_amt);
if (@typeOf(shift_amt).is_signed) {
if (shift_amt >= 0) {
return a >> casted_shift_amt;
} else {
return a << casted_shift_amt;
}
}
return a >> casted_shift_amt;
}
test "math.shr" {
assert(shr(u8, 0b11111111, usize(3)) == 0b00011111);
assert(shr(u8, 0b11111111, usize(8)) == 0);
assert(shr(u8, 0b11111111, usize(9)) == 0);
assert(shr(u8, 0b11111111, isize(-2)) == 0b11111100);
}
pub fn Log2Int(comptime T: type) -> type {
@IntType(false, log2(T.bit_count))
}
@ -237,7 +285,7 @@ fn testOverflow() {
assert(%%mul(i32, 3, 4) == 12);
assert(%%add(i32, 3, 4) == 7);
assert(%%sub(i32, 3, 4) == -1);
assert(%%shl(i32, 0b11, 4) == 0b110000);
assert(%%shlExact(i32, 0b11, 4) == 0b110000);
}