    //! This module provides functions for working conveniently with SIMD (Single Instruction; Multiple Data),
    //! which may offer a potential boost in performance on some targets by performing the same operations on
    //! multiple elements at once.
    //! Please be aware that some functions are known to not work on MIPS.

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

    pub fn suggestVectorSizeForCpu(comptime T: type, comptime cpu: std.Target.Cpu) ?usize {
        // This is guesswork, if you have better suggestions can add it or edit the current here

        // This can run in comptime only, but stage 1 fails at it, stage 2 can understand it

        const element_bit_size = @max(8, std.math.ceilPowerOfTwo(u16, @bitSizeOf(T)) catch unreachable);
        const vector_bit_size: u16 = blk: {
            if (cpu.arch.isX86()) {
                if (T == bool and std.Target.x86.featureSetHas(.prefer_mask_registers)) return 64;
                if (std.Target.x86.featureSetHas(cpu.features, .avx512f) and !std.Target.x86.featureSetHasAny(cpu.features, .{ .prefer_256_bit, .prefer_128_bit })) break :blk 512;
                if (std.Target.x86.featureSetHasAny(cpu.features, .{ .prefer_256_bit, .avx2 }) and !std.Target.x86.featureSetHas(cpu.features, .prefer_128_bit)) break :blk 256;
                if (std.Target.x86.featureSetHas(cpu.features, .sse)) break :blk 128;
                if (std.Target.x86.featureSetHasAny(cpu.features, .{ .mmx, .@"3dnow" })) break :blk 64;
            } else if (cpu.arch.isARM()) {
                if (std.Target.arm.featureSetHas(cpu.features, .neon)) break :blk 128;
            } else if (cpu.arch.isAARCH64()) {
                // SVE allows up to 2048 bits in the specification, as of 2022 the most powerful machine has implemented 512-bit

                // I think is safer to just be on 128 until is more common

                // TODO: Check on this return when bigger values are more common

                if (std.Target.aarch64.featureSetHas(cpu.features, .sve)) break :blk 128;
                if (std.Target.aarch64.featureSetHas(cpu.features, .neon)) break :blk 128;
            } else if (cpu.arch.isPPC() or cpu.arch.isPPC64()) {
                if (std.Target.powerpc.featureSetHas(cpu.features, .altivec)) break :blk 128;
            } else if (cpu.arch.isMIPS()) {
                if (std.Target.mips.featureSetHas(cpu.features, .msa)) break :blk 128;
                // TODO: Test MIPS capability to handle bigger vectors

                //       In theory MDMX and by extension mips3d have 32 registers of 64 bits which can use in parallel

                //       for multiple processing, but I don't know what's optimal here, if using

                //       the 2048 bits or using just 64 per vector or something in between

                if (std.Target.mips.featureSetHas(cpu.features, std.Target.mips.Feature.mips3d)) break :blk 64;
            } else if (cpu.arch.isRISCV()) {
                // in risc-v the Vector Extension allows configurable vector sizes, but a standard size of 128 is a safe estimate

                if (std.Target.riscv.featureSetHas(cpu.features, .v)) break :blk 128;
            } else if (cpu.arch.isSPARC()) {
                // TODO: Test Sparc capability to handle bigger vectors

                //       In theory Sparc have 32 registers of 64 bits which can use in parallel

                //       for multiple processing, but I don't know what's optimal here, if using

                //       the 2048 bits or using just 64 per vector or something in between

                if (std.Target.sparc.featureSetHasAny(cpu.features, .{ .vis, .vis2, .vis3 })) break :blk 64;
            }
            return null;
        };
        if (vector_bit_size <= element_bit_size) return null;

        return @divExact(vector_bit_size, element_bit_size);
    }

    /// Suggests a target-dependant vector size for a given type, or null if scalars are recommended.
    /// Not yet implemented for every CPU architecture.
    pub fn suggestVectorSize(comptime T: type) ?usize {
        return suggestVectorSizeForCpu(T, builtin.cpu);
    }

    test "suggestVectorSizeForCpu works with signed and unsigned values" {
        comptime var cpu = std.Target.Cpu.baseline(std.Target.Cpu.Arch.x86_64);
        comptime cpu.features.addFeature(@enumToInt(std.Target.x86.Feature.avx512f));
        const signed_integer_size = suggestVectorSizeForCpu(i32, cpu).?;
        const unsigned_integer_size = suggestVectorSizeForCpu(u32, cpu).?;
        try std.testing.expectEqual(@as(usize, 16), unsigned_integer_size);
        try std.testing.expectEqual(@as(usize, 16), signed_integer_size);
    }

    fn vectorLength(comptime VectorType: type) comptime_int {
        return switch (@typeInfo(VectorType)) {
            .Vector => |info| info.len,
            .Array => |info| info.len,
            else => @compileError("Invalid type " ++ @typeName(VectorType)),
        };
    }

    /// Returns the smallest type of unsigned ints capable of indexing any element within the given vector type.
    pub fn VectorIndex(comptime VectorType: type) type {
        return std.math.IntFittingRange(0, vectorLength(VectorType) - 1);
    }

    /// Returns the smallest type of unsigned ints capable of holding the length of the given vector type.
    pub fn VectorCount(comptime VectorType: type) type {
        return std.math.IntFittingRange(0, vectorLength(VectorType));
    }

    /// Returns a vector containing the first `len` integers in order from 0 to `len`-1.
    /// For example, `iota(i32, 8)` will return a vector containing `.{0, 1, 2, 3, 4, 5, 6, 7}`.
    pub fn iota(comptime T: type, comptime len: usize) @Vector(len, T) {
        var out: [len]T = undefined;
        for (out) |*element, i| {
            element.* = switch (@typeInfo(T)) {
                .Int => @intCast(T, i),
                .Float => @intToFloat(T, i),
                else => @compileError("Can't use type " ++ @typeName(T) ++ " in iota."),
            };
        }
        return @as(@Vector(len, T), out);
    }

    /// Returns a vector containing the same elements as the input, but repeated until the desired length is reached.
    /// For example, `repeat(8, [_]u32{1, 2, 3})` will return a vector containing `.{1, 2, 3, 1, 2, 3, 1, 2}`.
    pub fn repeat(comptime len: usize, vec: anytype) @Vector(len, std.meta.Child(@TypeOf(vec))) {
        const Child = std.meta.Child(@TypeOf(vec));

        return @shuffle(Child, vec, undefined, iota(i32, len) % @splat(len, @intCast(i32, vectorLength(@TypeOf(vec)))));
    }

    /// Returns a vector containing all elements of the first vector at the lower indices followed by all elements of the second vector
    /// at the higher indices.
    pub fn join(a: anytype, b: anytype) @Vector(vectorLength(@TypeOf(a)) + vectorLength(@TypeOf(b)), std.meta.Child(@TypeOf(a))) {
        const Child = std.meta.Child(@TypeOf(a));
        const a_len = vectorLength(@TypeOf(a));
        const b_len = vectorLength(@TypeOf(b));

        return @shuffle(Child, a, b, @as([a_len]i32, iota(i32, a_len)) ++ @as([b_len]i32, ~iota(i32, b_len)));
    }

    /// Returns a vector whose elements alternates between those of each input vector.
    /// For example, `interlace(.{[4]u32{11, 12, 13, 14}, [4]u32{21, 22, 23, 24}})` returns a vector containing `.{11, 21, 12, 22, 13, 23, 14, 24}`.
    pub fn interlace(vecs: anytype) @Vector(vectorLength(@TypeOf(vecs[0])) * vecs.len, std.meta.Child(@TypeOf(vecs[0]))) {
        // interlace doesn't work on MIPS, for some reason.

        // Notes from earlier debug attempt:

        //  The indices are correct. The problem seems to be with the @shuffle builtin.

        //  On MIPS, the test that interlaces small_base gives { 0, 2, 0, 0, 64, 255, 248, 200, 0, 0 }.

        //  Calling this with two inputs seems to work fine, but I'll let the compile error trigger for all inputs, just to be safe.

        comptime if (builtin.cpu.arch.isMIPS()) @compileError("TODO: Find out why interlace() doesn't work on MIPS");

        const VecType = @TypeOf(vecs[0]);
        const vecs_arr = @as([vecs.len]VecType, vecs);
        const Child = std.meta.Child(@TypeOf(vecs_arr[0]));

        if (vecs_arr.len == 1) return vecs_arr[0];

        const a_vec_count = (1 + vecs_arr.len) >> 1;
        const b_vec_count = vecs_arr.len >> 1;

        const a = interlace(@ptrCast(*const [a_vec_count]VecType, vecs_arr[0..a_vec_count]).*);
        const b = interlace(@ptrCast(*const [b_vec_count]VecType, vecs_arr[a_vec_count..]).*);

        const a_len = vectorLength(@TypeOf(a));
        const b_len = vectorLength(@TypeOf(b));
        const len = a_len + b_len;

        const indices = comptime blk: {
            const count_up = iota(i32, len);
            const cycle = @divFloor(count_up, @splat(len, @intCast(i32, vecs_arr.len)));
            const select_mask = repeat(len, join(@splat(a_vec_count, true), @splat(b_vec_count, false)));
            const a_indices = count_up - cycle * @splat(len, @intCast(i32, b_vec_count));
            const b_indices = shiftElementsRight(count_up - cycle * @splat(len, @intCast(i32, a_vec_count)), a_vec_count, 0);
            break :blk @select(i32, select_mask, a_indices, ~b_indices);
        };

        return @shuffle(Child, a, b, indices);
    }

    /// The contents of `interlaced` is evenly split between vec_count vectors that are returned as an array. They "take turns",
    /// recieving one element from `interlaced` at a time.
    pub fn deinterlace(
        comptime vec_count: usize,
        interlaced: anytype,
    ) [vec_count]@Vector(
        vectorLength(@TypeOf(interlaced)) / vec_count,
        std.meta.Child(@TypeOf(interlaced)),
    ) {
        const vec_len = vectorLength(@TypeOf(interlaced)) / vec_count;
        const Child = std.meta.Child(@TypeOf(interlaced));

        var out: [vec_count]@Vector(vec_len, Child) = undefined;

        comptime var i: usize = 0; // for-loops don't work for this, apparently.

        inline while (i < out.len) : (i += 1) {
            const indices = comptime iota(i32, vec_len) * @splat(vec_len, @intCast(i32, vec_count)) + @splat(vec_len, @intCast(i32, i));
            out[i] = @shuffle(Child, interlaced, undefined, indices);
        }

        return out;
    }

    pub fn extract(
        vec: anytype,
        comptime first: VectorIndex(@TypeOf(vec)),
        comptime count: VectorCount(@TypeOf(vec)),
    ) @Vector(count, std.meta.Child(@TypeOf(vec))) {
        const Child = std.meta.Child(@TypeOf(vec));
        const len = vectorLength(@TypeOf(vec));

        std.debug.assert(@intCast(comptime_int, first) + @intCast(comptime_int, count) <= len);

        return @shuffle(Child, vec, undefined, iota(i32, count) + @splat(count, @intCast(i32, first)));
    }

    test "vector patterns" {
        if ((builtin.zig_backend == .stage1 or builtin.zig_backend == .stage2_llvm) and
            builtin.cpu.arch == .aarch64)
        {
            // https://github.com/ziglang/zig/issues/12012

            return error.SkipZigTest;
        }
        const base = @Vector(4, u32){ 10, 20, 30, 40 };
        const other_base = @Vector(4, u32){ 55, 66, 77, 88 };

        const small_bases = [5]@Vector(2, u8){
            @Vector(2, u8){ 0, 1 },
            @Vector(2, u8){ 2, 3 },
            @Vector(2, u8){ 4, 5 },
            @Vector(2, u8){ 6, 7 },
            @Vector(2, u8){ 8, 9 },
        };

        try std.testing.expectEqual([6]u32{ 10, 20, 30, 40, 10, 20 }, repeat(6, base));
        try std.testing.expectEqual([8]u32{ 10, 20, 30, 40, 55, 66, 77, 88 }, join(base, other_base));
        try std.testing.expectEqual([2]u32{ 20, 30 }, extract(base, 1, 2));

        if (comptime !builtin.cpu.arch.isMIPS()) {
            try std.testing.expectEqual([8]u32{ 10, 55, 20, 66, 30, 77, 40, 88 }, interlace(.{ base, other_base }));

            const small_braid = interlace(small_bases);
            try std.testing.expectEqual([10]u8{ 0, 2, 4, 6, 8, 1, 3, 5, 7, 9 }, small_braid);
            try std.testing.expectEqual(small_bases, deinterlace(small_bases.len, small_braid));
        }
    }

    /// Joins two vectors, shifts them leftwards (towards lower indices) and extracts the leftmost elements into a vector the size of a and b.
    pub fn mergeShift(a: anytype, b: anytype, comptime shift: VectorCount(@TypeOf(a, b))) @TypeOf(a, b) {
        const len = vectorLength(@TypeOf(a, b));

        return extract(join(a, b), shift, len);
    }

    /// Elements are shifted rightwards (towards higher indices). New elements are added to the left, and the rightmost elements are cut off
    /// so that the size of the vector stays the same.
    pub fn shiftElementsRight(vec: anytype, comptime amount: VectorCount(@TypeOf(vec)), shift_in: std.meta.Child(@TypeOf(vec))) @TypeOf(vec) {
        // It may be possible to implement shifts and rotates with a runtime-friendly slice of two joined vectors, as the length of the

        // slice would be comptime-known. This would permit vector shifts and rotates by a non-comptime-known amount.

        // However, I am unsure whether compiler optimizations would handle that well enough on all platforms.

        const len = vectorLength(@TypeOf(vec));

        return mergeShift(@splat(len, shift_in), vec, len - amount);
    }

    /// Elements are shifted leftwards (towards lower indices). New elements are added to the right, and the leftmost elements are cut off
    /// so that no elements with indices below 0 remain.
    pub fn shiftElementsLeft(vec: anytype, comptime amount: VectorCount(@TypeOf(vec)), shift_in: std.meta.Child(@TypeOf(vec))) @TypeOf(vec) {
        const len = vectorLength(@TypeOf(vec));

        return mergeShift(vec, @splat(len, shift_in), amount);
    }

    /// Elements are shifted leftwards (towards lower indices). Elements that leave to the left will reappear to the right in the same order.
    pub fn rotateElementsLeft(vec: anytype, comptime amount: VectorCount(@TypeOf(vec))) @TypeOf(vec) {
        return mergeShift(vec, vec, amount);
    }

    /// Elements are shifted rightwards (towards higher indices). Elements that leave to the right will reappear to the left in the same order.
    pub fn rotateElementsRight(vec: anytype, comptime amount: VectorCount(@TypeOf(vec))) @TypeOf(vec) {
        return rotateElementsLeft(vec, vectorLength(@TypeOf(vec)) - amount);
    }

    pub fn reverseOrder(vec: anytype) @TypeOf(vec) {
        const Child = std.meta.Child(@TypeOf(vec));
        const len = vectorLength(@TypeOf(vec));

        return @shuffle(Child, vec, undefined, @splat(len, @intCast(i32, len) - 1) - iota(i32, len));
    }

    test "vector shifting" {
        const base = @Vector(4, u32){ 10, 20, 30, 40 };

        try std.testing.expectEqual([4]u32{ 30, 40, 999, 999 }, shiftElementsLeft(base, 2, 999));
        try std.testing.expectEqual([4]u32{ 999, 999, 10, 20 }, shiftElementsRight(base, 2, 999));
        try std.testing.expectEqual([4]u32{ 20, 30, 40, 10 }, rotateElementsLeft(base, 1));
        try std.testing.expectEqual([4]u32{ 40, 10, 20, 30 }, rotateElementsRight(base, 1));
        try std.testing.expectEqual([4]u32{ 40, 30, 20, 10 }, reverseOrder(base));
    }

    pub fn firstTrue(vec: anytype) ?VectorIndex(@TypeOf(vec)) {
        const len = vectorLength(@TypeOf(vec));
        const IndexInt = VectorIndex(@TypeOf(vec));

        if (!@reduce(.Or, vec)) {
            return null;
        }
        const indices = @select(IndexInt, vec, iota(IndexInt, len), @splat(len, ~@as(IndexInt, 0)));
        return @reduce(.Min, indices);
    }

    pub fn lastTrue(vec: anytype) ?VectorIndex(@TypeOf(vec)) {
        const len = vectorLength(@TypeOf(vec));
        const IndexInt = VectorIndex(@TypeOf(vec));

        if (!@reduce(.Or, vec)) {
            return null;
        }
        const indices = @select(IndexInt, vec, iota(IndexInt, len), @splat(len, @as(IndexInt, 0)));
        return @reduce(.Max, indices);
    }

    pub fn countTrues(vec: anytype) VectorCount(@TypeOf(vec)) {
        const len = vectorLength(@TypeOf(vec));
        const CountIntType = VectorCount(@TypeOf(vec));

        const one_if_true = @select(CountIntType, vec, @splat(len, @as(CountIntType, 1)), @splat(len, @as(CountIntType, 0)));
        return @reduce(.Add, one_if_true);
    }

    pub fn firstIndexOfValue(vec: anytype, value: std.meta.Child(@TypeOf(vec))) ?VectorIndex(@TypeOf(vec)) {
        const len = vectorLength(@TypeOf(vec));

        return firstTrue(vec == @splat(len, value));
    }

    pub fn lastIndexOfValue(vec: anytype, value: std.meta.Child(@TypeOf(vec))) ?VectorIndex(@TypeOf(vec)) {
        const len = vectorLength(@TypeOf(vec));

        return lastTrue(vec == @splat(len, value));
    }

    pub fn countElementsWithValue(vec: anytype, value: std.meta.Child(@TypeOf(vec))) VectorCount(@TypeOf(vec)) {
        const len = vectorLength(@TypeOf(vec));

        return countTrues(vec == @splat(len, value));
    }

    test "vector searching" {
        const base = @Vector(8, u32){ 6, 4, 7, 4, 4, 2, 3, 7 };

        try std.testing.expectEqual(@as(?u3, 1), firstIndexOfValue(base, 4));
        try std.testing.expectEqual(@as(?u3, 4), lastIndexOfValue(base, 4));
        try std.testing.expectEqual(@as(?u3, null), lastIndexOfValue(base, 99));
        try std.testing.expectEqual(@as(u4, 3), countElementsWithValue(base, 4));
    }

    /// Same as prefixScan, but with a user-provided, mathematically associative function.
    pub fn prefixScanWithFunc(
        comptime hop: isize,
        vec: anytype,
        /// The error type that `func` might return. Set this to `void` if `func` doesn't return an error union.
        comptime ErrorType: type,
        comptime func: fn (@TypeOf(vec), @TypeOf(vec)) if (ErrorType == void) @TypeOf(vec) else ErrorType!@TypeOf(vec),
        /// When one operand of the operation performed by `func` is this value, the result must equal the other operand.
        /// For example, this should be 0 for addition or 1 for multiplication.
        comptime identity: std.meta.Child(@TypeOf(vec)),
    ) if (ErrorType == void) @TypeOf(vec) else ErrorType!@TypeOf(vec) {
        // I haven't debugged this, but it might be a cousin of sorts to what's going on with interlace.

        comptime if (builtin.cpu.arch.isMIPS()) @compileError("TODO: Find out why prefixScan doesn't work on MIPS");

        const len = vectorLength(@TypeOf(vec));

        if (hop == 0) @compileError("hop can not be 0; you'd be going nowhere forever!");
        const abs_hop = if (hop < 0) -hop else hop;

        var acc = vec;
        comptime var i = 0;
        inline while ((abs_hop << i) < len) : (i += 1) {
            const shifted = if (hop < 0) shiftElementsLeft(acc, abs_hop << i, identity) else shiftElementsRight(acc, abs_hop << i, identity);

            acc = if (ErrorType == void) func(acc, shifted) else try func(acc, shifted);
        }
        return acc;
    }

    /// Returns a vector whose elements are the result of performing the specified operation on the corresponding
    /// element of the input vector and every hop'th element that came before it (or after, if hop is negative).
    /// Supports the same operations as the @reduce() builtin. Takes O(logN) to compute.
    /// The scan is not linear, which may affect floating point errors. This may affect the determinism of
    /// algorithms that use this function.
    pub fn prefixScan(comptime op: std.builtin.ReduceOp, comptime hop: isize, vec: anytype) @TypeOf(vec) {
        const VecType = @TypeOf(vec);
        const Child = std.meta.Child(VecType);
        const len = vectorLength(VecType);

        const identity = comptime switch (@typeInfo(Child)) {
            .Bool => switch (op) {
                .Or, .Xor => false,
                .And => true,
                else => @compileError("Invalid prefixScan operation " ++ @tagName(op) ++ " for vector of booleans."),
            },
            .Int => switch (op) {
                .Max => std.math.minInt(Child),
                .Add, .Or, .Xor => 0,
                .Mul => 1,
                .And, .Min => std.math.maxInt(Child),
            },
            .Float => switch (op) {
                .Max => -std.math.inf(Child),
                .Add => 0,
                .Mul => 1,
                .Min => std.math.inf(Child),
                else => @compileError("Invalid prefixScan operation " ++ @tagName(op) ++ " for vector of floats."),
            },
            else => @compileError("Invalid type " ++ @typeName(VecType) ++ " for prefixScan."),
        };

        const fn_container = struct {
            fn opFn(a: VecType, b: VecType) VecType {
                return if (Child == bool) switch (op) {
                    .And => @select(bool, a, b, @splat(len, false)),
                    .Or => @select(bool, a, @splat(len, true), b),
                    .Xor => a != b,
                    else => unreachable,
                } else switch (op) {
                    .And => a & b,
                    .Or => a | b,
                    .Xor => a ^ b,
                    .Add => a + b,
                    .Mul => a * b,
                    .Min => @min(a, b),
                    .Max => @max(a, b),
                };
            }
        };

        return prefixScanWithFunc(hop, vec, void, fn_container.opFn, identity);
    }

    test "vector prefix scan" {
        if (comptime builtin.cpu.arch.isMIPS()) {
            return error.SkipZigTest;
        }

        if (builtin.zig_backend == .stage1 or builtin.zig_backend == .stage2_llvm) {
            // Regressed in LLVM 14:

            // https://github.com/llvm/llvm-project/issues/55522

            return error.SkipZigTest;
        }

        const int_base = @Vector(4, i32){ 11, 23, 9, -21 };
        const float_base = @Vector(4, f32){ 2, 0.5, -10, 6.54321 };
        const bool_base = @Vector(4, bool){ true, false, true, false };

        try std.testing.expectEqual(iota(u8, 32) + @splat(32, @as(u8, 1)), prefixScan(.Add, 1, @splat(32, @as(u8, 1))));
        try std.testing.expectEqual(@Vector(4, i32){ 11, 3, 1, 1 }, prefixScan(.And, 1, int_base));
        try std.testing.expectEqual(@Vector(4, i32){ 11, 31, 31, -1 }, prefixScan(.Or, 1, int_base));
        try std.testing.expectEqual(@Vector(4, i32){ 11, 28, 21, -2 }, prefixScan(.Xor, 1, int_base));
        try std.testing.expectEqual(@Vector(4, i32){ 11, 34, 43, 22 }, prefixScan(.Add, 1, int_base));
        try std.testing.expectEqual(@Vector(4, i32){ 11, 253, 2277, -47817 }, prefixScan(.Mul, 1, int_base));
        try std.testing.expectEqual(@Vector(4, i32){ 11, 11, 9, -21 }, prefixScan(.Min, 1, int_base));
        try std.testing.expectEqual(@Vector(4, i32){ 11, 23, 23, 23 }, prefixScan(.Max, 1, int_base));

        // Trying to predict all inaccuracies when adding and multiplying floats with prefixScans would be a mess, so we don't test those.

        try std.testing.expectEqual(@Vector(4, f32){ 2, 0.5, -10, -10 }, prefixScan(.Min, 1, float_base));
        try std.testing.expectEqual(@Vector(4, f32){ 2, 2, 2, 6.54321 }, prefixScan(.Max, 1, float_base));

        try std.testing.expectEqual(@Vector(4, bool){ true, true, false, false }, prefixScan(.Xor, 1, bool_base));
        try std.testing.expectEqual(@Vector(4, bool){ true, true, true, true }, prefixScan(.Or, 1, bool_base));
        try std.testing.expectEqual(@Vector(4, bool){ true, false, false, false }, prefixScan(.And, 1, bool_base));

        try std.testing.expectEqual(@Vector(4, i32){ 11, 23, 20, 2 }, prefixScan(.Add, 2, int_base));
        try std.testing.expectEqual(@Vector(4, i32){ 22, 11, -12, -21 }, prefixScan(.Add, -1, int_base));
        try std.testing.expectEqual(@Vector(4, i32){ 11, 23, 9, -10 }, prefixScan(.Add, 3, int_base));
    }