const builtin = @import("builtin");
const std = @import("std");

const endian = builtin.target.cpu.arch.endian();

const min = std.math.minInt(i52) + 1;
const max = std.math.maxInt(i52) - 1;

export fn checkValidRange(int: i64) u8 {
    return if (min < int and int < max) 1 else 0;
}

export fn checkValidRange2(int: i64) u8 {
    const x: u64 = @bitCast(int);
    return if (if (int < 0)
        x >> 51 == std.math.maxInt(u13)
    else
        x >> 51 == 0) 1 else 0;
}

export fn isFixnum(v: u64) u8 {
    const expt = v >> 52;
    const rest = v << 13;
    return if (expt == 0xfff and rest != 0) 1 else 0;
}

export fn isFixnum2(v: u64) u8 {
    const hi: u14 = @intCast(v >> 50);
    return if (hi == (0xfff << 2 | 0b01)) 1 else 0;
}

export fn sstrLen(x: u64) u8 {
    const bytes: @Vector(8, u8) = @bitCast(x);
    const nulls: @Vector(8, u8) = @splat(0);
    const comps: u8 = @bitCast(bytes == nulls);
    // Two bits will always be 0, since the actual short string starts at the
    // third byte; third lowest or third highest depending on endianness.  So,
    // depending on endianness, either cut off the two leading bits and ensure
    // that the second-last is set, or ensure that the second highest set, to
    // limit the length to 6.
    return switch (endian) {
        .big => @clz(comps << 2 | 2),
        .little => @ctz(comps | 64),
    };
}

const positive_mask: u64 = 0xfff7ffffffffffff;

fn unpackNegative(v: u64) i64 {
    return @bitCast(v);
}

fn unpackPositive(v: u64) i64 {
    const uint: u64 = @bitCast(v);
    return @bitCast(uint ^ positive_mask);
}

fn packNegative(int: i64) u64 {
    return @bitCast(int);
}

fn packPositive(int: i64) u64 {
    const uint: u64 = @bitCast(int);
    return @bitCast(uint ^ positive_mask);
}

export fn packFx(int: i64) u64 {
    if (int < 0) {
        return packNegative(int);
    } else {
        return packPositive(int);
    }
}

export fn packSstr(len: usize, s: [*]const u8) u64 {
    var v: u64 = 0xfff3000000000000;
    const buf: *[8]u8 = @ptrCast(&v);
    @memcpy(buf[0 .. 0 + len], s);
    return v;
}

pub fn packSstrTest(buf: [*]const u64, len: usize) ?Value {
    if (len > 6) return null;

    const v = switch (endian) {
        .big => @byteSwap(buf[0]),
        .little => buf[0],
    };

    const shift: u6 = 8 * @as(u6, @intCast(len));
    const mask = @as(u64, std.math.maxInt(u64)) << shift;
    const s = Value{ .sstr = .{ .bytes = @intCast(v & ~mask) } };

    // This effectively checks if there were any NUL bytes:
    if (len != value.sstrLen(&s)) return null;

    return s;
}

pub fn packSstrStatic(comptime s: []const u8) Value {
    var val: u64 = undefined;
    const buf: [*]align(8) u8 = @ptrCast(&val);
    @memcpy(buf, s);
    return pack(@ptrCast(buf), s.len).?;
}