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Add big int fits function #1279

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Jul 22, 2018
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Add big int fits function #1279

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39 changes: 39 additions & 0 deletions std/math/big/int.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -150,6 +150,18 @@ pub const Int = struct {
return bits;
}

pub fn fits(self: Int, comptime T: type) bool {
if (self.eqZero()) {
return true;
}
if (!T.is_signed and !self.positive) {
return false;
}

const req_bits = self.bitCountTwosComp() + @boolToInt(self.positive and T.is_signed);
return T.bit_count >= req_bits;
}

// Returns the approximate size of the integer in the given base. Negative values accomodate for
// the minus sign. This is used for determining the number of characters needed to print the
// value. It is inexact and will exceed the given value by 1-2 digits.
Expand Down Expand Up @@ -1217,6 +1229,33 @@ test "big.int bitcount/to" {
debug.assert((try a.to(i9)) == -129);
}

test "big.int fits" {
var a = try Int.init(al);

try a.set(0);
debug.assert(a.fits(u0));
debug.assert(a.fits(i0));

try a.set(255);
debug.assert(!a.fits(u0));
debug.assert(!a.fits(u1));
debug.assert(!a.fits(i8));
debug.assert(a.fits(u8));
debug.assert(a.fits(u9));
debug.assert(a.fits(i9));

try a.set(-128);
debug.assert(!a.fits(i7));
debug.assert(a.fits(i8));
debug.assert(a.fits(i9));
debug.assert(!a.fits(u9));

try a.set(0x1ffffffffeeeeeeee);
debug.assert(!a.fits(u32));
debug.assert(!a.fits(u64));
debug.assert(a.fits(u65));
}

test "big.int string set" {
var a = try Int.init(al);
try a.setString(10, "120317241209124781241290847124");
Expand Down