    const std = @import("std.zig");
    const assert = std.debug.assert;
    const mem = std.mem;
    const testing = std.testing;

    /// A structure with an array and a length, that can be used as a slice.
    ///
    /// Useful to pass around small arrays whose exact size is only known at
    /// runtime, but whose maximum size is known at comptime, without requiring
    /// an `Allocator`.
    ///
    /// ```zig
    /// var actual_size = 32;
    /// var a = try BoundedArray(u8, 64).init(actual_size);
    /// var slice = a.slice(); // a slice of the 64-byte array
    /// var a_clone = a; // creates a copy - the structure doesn't use any internal pointers
    /// ```
    pub fn BoundedArray(comptime T: type, comptime buffer_capacity: usize) type {
        return struct {
            const Self = @This();
            buffer: [buffer_capacity]T = undefined,
            len: usize = 0,

            /// Set the actual length of the slice.
            /// Returns error.Overflow if it exceeds the length of the backing array.
            pub fn init(len: usize) error{Overflow}!Self {
                if (len > buffer_capacity) return error.Overflow;
                return Self{ .len = len };
            }

            /// View the internal array as a slice whose size was previously set.
            pub fn slice(self: anytype) mem.Span(@TypeOf(&self.buffer)) {
                return self.buffer[0..self.len];
            }

            /// View the internal array as a constant slice whose size was previously set.
            pub fn constSlice(self: *const Self) []const T {
                return self.slice();
            }

            /// Adjust the slice's length to `len`.
            /// Does not initialize added items if any.
            pub fn resize(self: *Self, len: usize) error{Overflow}!void {
                if (len > buffer_capacity) return error.Overflow;
                self.len = len;
            }

            /// Copy the content of an existing slice.
            pub fn fromSlice(m: []const T) error{Overflow}!Self {
                var list = try init(m.len);
                std.mem.copy(T, list.slice(), m);
                return list;
            }

            /// Return the element at index `i` of the slice.
            pub fn get(self: Self, i: usize) T {
                return self.constSlice()[i];
            }

            /// Set the value of the element at index `i` of the slice.
            pub fn set(self: *Self, i: usize, item: T) void {
                self.slice()[i] = item;
            }

            /// Return the maximum length of a slice.
            pub fn capacity(self: Self) usize {
                return self.buffer.len;
            }

            /// Check that the slice can hold at least `additional_count` items.
            pub fn ensureUnusedCapacity(self: Self, additional_count: usize) error{Overflow}!void {
                if (self.len + additional_count > buffer_capacity) {
                    return error.Overflow;
                }
            }

            /// Increase length by 1, returning a pointer to the new item.
            pub fn addOne(self: *Self) error{Overflow}!*T {
                try self.ensureUnusedCapacity(1);
                return self.addOneAssumeCapacity();
            }

            /// Increase length by 1, returning pointer to the new item.
            /// Asserts that there is space for the new item.
            pub fn addOneAssumeCapacity(self: *Self) *T {
                assert(self.len < buffer_capacity);
                self.len += 1;
                return &self.slice()[self.len - 1];
            }

            /// Resize the slice, adding `n` new elements, which have `undefined` values.
            /// The return value is a slice pointing to the uninitialized elements.
            pub fn addManyAsArray(self: *Self, comptime n: usize) error{Overflow}!*[n]T {
                const prev_len = self.len;
                try self.resize(self.len + n);
                return self.slice()[prev_len..][0..n];
            }

            /// Remove and return the last element from the slice.
            /// Asserts the slice has at least one item.
            pub fn pop(self: *Self) T {
                const item = self.get(self.len - 1);
                self.len -= 1;
                return item;
            }

            /// Remove and return the last element from the slice, or
            /// return `null` if the slice is empty.
            pub fn popOrNull(self: *Self) ?T {
                return if (self.len == 0) null else self.pop();
            }

            /// Return a slice of only the extra capacity after items.
            /// This can be useful for writing directly into it.
            /// Note that such an operation must be followed up with a
            /// call to `resize()`
            pub fn unusedCapacitySlice(self: *Self) []T {
                return self.buffer[self.len..];
            }

            /// Insert `item` at index `i` by moving `slice[n .. slice.len]` to make room.
            /// This operation is O(N).
            pub fn insert(
                self: *Self,
                i: usize,
                item: T,
            ) error{Overflow}!void {
                if (i > self.len) {
                    return error.Overflow;
                }
                _ = try self.addOne();
                var s = self.slice();
                mem.copyBackwards(T, s[i + 1 .. s.len], s[i .. s.len - 1]);
                self.buffer[i] = item;
            }

            /// Insert slice `items` at index `i` by moving `slice[i .. slice.len]` to make room.
            /// This operation is O(N).
            pub fn insertSlice(self: *Self, i: usize, items: []const T) error{Overflow}!void {
                try self.ensureUnusedCapacity(items.len);
                self.len += items.len;
                mem.copyBackwards(T, self.slice()[i + items.len .. self.len], self.constSlice()[i .. self.len - items.len]);
                mem.copy(T, self.slice()[i .. i + items.len], items);
            }

            /// Replace range of elements `slice[start..start+len]` with `new_items`.
            /// Grows slice if `len < new_items.len`.
            /// Shrinks slice if `len > new_items.len`.
            pub fn replaceRange(
                self: *Self,
                start: usize,
                len: usize,
                new_items: []const T,
            ) error{Overflow}!void {
                const after_range = start + len;
                var range = self.slice()[start..after_range];

                if (range.len == new_items.len) {
                    mem.copy(T, range, new_items);
                } else if (range.len < new_items.len) {
                    const first = new_items[0..range.len];
                    const rest = new_items[range.len..];
                    mem.copy(T, range, first);
                    try self.insertSlice(after_range, rest);
                } else {
                    mem.copy(T, range, new_items);
                    const after_subrange = start + new_items.len;
                    for (self.constSlice()[after_range..]) |item, i| {
                        self.slice()[after_subrange..][i] = item;
                    }
                    self.len -= len - new_items.len;
                }
            }

            /// Extend the slice by 1 element.
            pub fn append(self: *Self, item: T) error{Overflow}!void {
                const new_item_ptr = try self.addOne();
                new_item_ptr.* = item;
            }

            /// Extend the slice by 1 element, asserting the capacity is already
            /// enough to store the new item.
            pub fn appendAssumeCapacity(self: *Self, item: T) void {
                const new_item_ptr = self.addOneAssumeCapacity();
                new_item_ptr.* = item;
            }

            /// Remove the element at index `i`, shift elements after index
            /// `i` forward, and return the removed element.
            /// Asserts the slice has at least one item.
            /// This operation is O(N).
            pub fn orderedRemove(self: *Self, i: usize) T {
                const newlen = self.len - 1;
                if (newlen == i) return self.pop();
                const old_item = self.get(i);
                for (self.slice()[i..newlen]) |*b, j| b.* = self.get(i + 1 + j);
                self.set(newlen, undefined);
                self.len = newlen;
                return old_item;
            }

            /// Remove the element at the specified index and return it.
            /// The empty slot is filled from the end of the slice.
            /// This operation is O(1).
            pub fn swapRemove(self: *Self, i: usize) T {
                if (self.len - 1 == i) return self.pop();
                const old_item = self.get(i);
                self.set(i, self.pop());
                return old_item;
            }

            /// Append the slice of items to the slice.
            pub fn appendSlice(self: *Self, items: []const T) error{Overflow}!void {
                try self.ensureUnusedCapacity(items.len);
                self.appendSliceAssumeCapacity(items);
            }

            /// Append the slice of items to the slice, asserting the capacity is already
            /// enough to store the new items.
            pub fn appendSliceAssumeCapacity(self: *Self, items: []const T) void {
                const oldlen = self.len;
                self.len += items.len;
                mem.copy(T, self.slice()[oldlen..], items);
            }

            /// Append a value to the slice `n` times.
            /// Allocates more memory as necessary.
            pub fn appendNTimes(self: *Self, value: T, n: usize) error{Overflow}!void {
                const old_len = self.len;
                try self.resize(old_len + n);
                mem.set(T, self.slice()[old_len..self.len], value);
            }

            /// Append a value to the slice `n` times.
            /// Asserts the capacity is enough.
            pub fn appendNTimesAssumeCapacity(self: *Self, value: T, n: usize) void {
                const old_len = self.len;
                self.len += n;
                assert(self.len <= buffer_capacity);
                mem.set(T, self.slice()[old_len..self.len], value);
            }

            pub const Writer = if (T != u8)
                @compileError("The Writer interface is only defined for BoundedArray(u8, ...) " ++
                    "but the given type is BoundedArray(" ++ @typeName(T) ++ ", ...)")
            else
                std.io.Writer(*Self, error{Overflow}, appendWrite);

            /// Initializes a writer which will write into the array.
            pub fn writer(self: *Self) Writer {
                return .{ .context = self };
            }

            /// Same as `appendSlice` except it returns the number of bytes written, which is always the same
            /// as `m.len`. The purpose of this function existing is to match `std.io.Writer` API.
            fn appendWrite(self: *Self, m: []const u8) error{Overflow}!usize {
                try self.appendSlice(m);
                return m.len;
            }
        };
    }

    test "BoundedArray" {
        var a = try BoundedArray(u8, 64).init(32);

        try testing.expectEqual(a.capacity(), 64);
        try testing.expectEqual(a.slice().len, 32);
        try testing.expectEqual(a.constSlice().len, 32);

        try a.resize(48);
        try testing.expectEqual(a.len, 48);

        const x = [_]u8{1} ** 10;
        a = try BoundedArray(u8, 64).fromSlice(&x);
        try testing.expectEqualSlices(u8, &x, a.constSlice());

        var a2 = a;
        try testing.expectEqualSlices(u8, a.constSlice(), a2.constSlice());
        a2.set(0, 0);
        try testing.expect(a.get(0) != a2.get(0));

        try testing.expectError(error.Overflow, a.resize(100));
        try testing.expectError(error.Overflow, BoundedArray(u8, x.len - 1).fromSlice(&x));

        try a.resize(0);
        try a.ensureUnusedCapacity(a.capacity());
        (try a.addOne()).* = 0;
        try a.ensureUnusedCapacity(a.capacity() - 1);
        try testing.expectEqual(a.len, 1);

        const uninitialized = try a.addManyAsArray(4);
        try testing.expectEqual(uninitialized.len, 4);
        try testing.expectEqual(a.len, 5);

        try a.append(0xff);
        try testing.expectEqual(a.len, 6);
        try testing.expectEqual(a.pop(), 0xff);

        a.appendAssumeCapacity(0xff);
        try testing.expectEqual(a.len, 6);
        try testing.expectEqual(a.pop(), 0xff);

        try a.resize(1);
        try testing.expectEqual(a.popOrNull(), 0);
        try testing.expectEqual(a.popOrNull(), null);
        var unused = a.unusedCapacitySlice();
        mem.set(u8, unused[0..8], 2);
        unused[8] = 3;
        unused[9] = 4;
        try testing.expectEqual(unused.len, a.capacity());
        try a.resize(10);

        try a.insert(5, 0xaa);
        try testing.expectEqual(a.len, 11);
        try testing.expectEqual(a.get(5), 0xaa);
        try testing.expectEqual(a.get(9), 3);
        try testing.expectEqual(a.get(10), 4);

        try a.insert(11, 0xbb);
        try testing.expectEqual(a.len, 12);
        try testing.expectEqual(a.pop(), 0xbb);

        try a.appendSlice(&x);
        try testing.expectEqual(a.len, 11 + x.len);

        try a.appendNTimes(0xbb, 5);
        try testing.expectEqual(a.len, 11 + x.len + 5);
        try testing.expectEqual(a.pop(), 0xbb);

        a.appendNTimesAssumeCapacity(0xcc, 5);
        try testing.expectEqual(a.len, 11 + x.len + 5 - 1 + 5);
        try testing.expectEqual(a.pop(), 0xcc);

        try testing.expectEqual(a.len, 29);
        try a.replaceRange(1, 20, &x);
        try testing.expectEqual(a.len, 29 + x.len - 20);

        try a.insertSlice(0, &x);
        try testing.expectEqual(a.len, 29 + x.len - 20 + x.len);

        try a.replaceRange(1, 5, &x);
        try testing.expectEqual(a.len, 29 + x.len - 20 + x.len + x.len - 5);

        try a.append(10);
        try testing.expectEqual(a.pop(), 10);

        try a.append(20);
        const removed = a.orderedRemove(5);
        try testing.expectEqual(removed, 1);
        try testing.expectEqual(a.len, 34);

        a.set(0, 0xdd);
        a.set(a.len - 1, 0xee);
        const swapped = a.swapRemove(0);
        try testing.expectEqual(swapped, 0xdd);
        try testing.expectEqual(a.get(0), 0xee);

        while (a.popOrNull()) |_| {}
        const w = a.writer();
        const s = "hello, this is a test string";
        try w.writeAll(s);
        try testing.expectEqualStrings(s, a.constSlice());
    }