Function writeSignedFixed [src]

This is an "advanced" function. It allows one to use a fixed amount of memory to store an ILEB128. This defeats the entire purpose of using this data encoding; it will no longer use fewer bytes to store smaller numbers. The advantage of using a fixed width is that it makes fields have a predictable size and so depending on the use case this tradeoff can be worthwhile. An example use case of this is in emitting DWARF info where one wants to make a ILEB128 field "relocatable", meaning that it becomes possible to later go back and patch the number to be a different value without shifting all the following code.

Prototype

pub fn writeSignedFixed(comptime l: usize, ptr: *[l]u8, int: std.meta.Int(.signed, l * 7)) void

Parameters

l: usizeptr: *[l]u8int: std.meta.Int(.signed, l * 7)

Example

 test writeSignedFixed {
    {
        var buf: [4]u8 = undefined;
        writeSignedFixed(4, &buf, 0);
        try testing.expect((try test_read_ileb128(i64, &buf)) == 0);
    }
    {
        var buf: [4]u8 = undefined;
        writeSignedFixed(4, &buf, 1);
        try testing.expect((try test_read_ileb128(i64, &buf)) == 1);
    }
    {
        var buf: [4]u8 = undefined;
        writeSignedFixed(4, &buf, -1);
        try testing.expect((try test_read_ileb128(i64, &buf)) == -1);
    }
    {
        var buf: [4]u8 = undefined;
        writeSignedFixed(4, &buf, 1000);
        try testing.expect((try test_read_ileb128(i64, &buf)) == 1000);
    }
    {
        var buf: [4]u8 = undefined;
        writeSignedFixed(4, &buf, -1000);
        try testing.expect((try test_read_ileb128(i64, &buf)) == -1000);
    }
    {
        var buf: [4]u8 = undefined;
        writeSignedFixed(4, &buf, -10000000);
        try testing.expect((try test_read_ileb128(i64, &buf)) == -10000000);
    }
    {
        var buf: [4]u8 = undefined;
        writeSignedFixed(4, &buf, 10000000);
        try testing.expect((try test_read_ileb128(i64, &buf)) == 10000000);
    }
}

Source

  pub fn writeSignedFixed(comptime l: usize, ptr: *[l]u8, int: std.meta.Int(.signed, l * 7)) void {
    const T = @TypeOf(int);
    const U = if (@typeInfo(T).int.bits < 8) u8 else T;
    var value: U = @intCast(int);

    comptime var i = 0;
    inline while (i < (l - 1)) : (i += 1) {
        const byte: u8 = @bitCast(@as(i8, @truncate(value)) | -0b1000_0000);
        value >>= 7;
        ptr[i] = byte;
    }
    ptr[i] = @as(u7, @bitCast(@as(i7, @truncate(value))));
}  