//! Interface for Element's which describe the contents of a `Container`.
const std = @import("std");
const s = @import("size.zig");

const Container = @import("container.zig").Container;
const Cell = @import("cell.zig");
const Position = s.Position;
const Size = s.Size;

pub fn Element(Event: type) type {
    return struct {
        ptr: *anyopaque = undefined,
        vtable: *const VTable = &.{},

        pub const VTable = struct {
            handle: ?*const fn (ctx: *anyopaque, event: Event) anyerror!void = null,
            content: ?*const fn (ctx: *anyopaque, cells: []Cell, size: Size) anyerror!void = null,
        };

        /// Handle the received event. The event is one of the user provided
        /// events or a system event, with the exception of the `.resize`
        /// `Event` as every `Container` already handles that event.
        ///
        /// In case of user errors this function should return an error. This
        /// error may then be used by the application to display information
        /// about the user error.
        pub inline fn handle(this: @This(), event: Event) !void {
            if (this.vtable.handle) |handle_fn|
                try handle_fn(this.ptr, event);
        }

        /// Write content into the `cells` of the `Container`. The associated
        /// `cells` slice has the size of (`size.cols * size.rows`). The
        /// renderer will know where to place the contents on the screen.
        ///
        /// This function should only fail with an error if the error is
        /// non-recoverable (i.e. an allocation error, system error, etc.).
        /// Otherwise user specific errors should be caught using the `handle`
        /// function before the rendering of the `Container` happens.
        pub inline fn content(this: @This(), cells: []Cell, size: Size) !void {
            if (this.vtable.content) |content_fn|
                try content_fn(this.ptr, cells, size);
        }
    };
}

/// This is an empty template implementation for an Element type which `zterm` may provide.
///
/// TODO: Should elements need to be composible with each other, such that they may build complexer outputs?
/// - the goal would rather be to have re-usable parts of handlers and/or content functions which serve similar functionalities.
/// - composible through empty `Container`'s? -> make sense for the `Scrollable` `Element`
pub fn Template(Event: type) type {
    return packed struct {
        pub fn element(this: *@This()) Element(Event) {
            return .{
                .ptr = this,
                .vtable = &.{
                    .handle = handle,
                    .content = content,
                },
            };
        }

        fn handle(ctx: *anyopaque, event: Event) !void {
            const this: *@This() = @ptrCast(@alignCast(ctx));
            _ = this;
            switch (event) {
                else => {},
            }
        }

        fn content(ctx: *anyopaque, cells: []Cell, size: Size) !void {
            const this: *@This() = @ptrCast(@alignCast(ctx));
            std.debug.assert(cells.len == @as(usize, size.cols) * @as(usize, size.rows));
            _ = this;
        }
    };
}

pub fn Scrollable(Event: type) type {
    return struct {
        /// `Size` of the actual contents where the anchor and the size is
        /// representing the size and location on screen.
        size: Size = .{},
        /// Anchor
        anchor: Position = .{},
        /// The actual container, that is *scrollable*
        container: Container(Event),
        /// Enable horizontal scrolling. This also renders a scrollbar (along the bottom of the viewport).
        horizontal: bool = false,
        /// Enable vertical scrolling. This also renders a scrollbar (along the right of the viewport).
        vertical: bool = false,

        pub fn element(this: *@This()) Element(Event) {
            return .{
                .ptr = this,
                .vtable = &.{
                    .handle = handle,
                    .content = content,
                },
            };
        }

        fn handle(ctx: *anyopaque, event: Event) !void {
            const this: *@This() = @ptrCast(@alignCast(ctx));
            switch (event) {
                .init => try this.container.handle(event),
                // TODO: emit `.resize` event for the container to set the size for the scrollable `Container`
                // - how would I determine the required or necessary `Size`?
                .resize => |size| {
                    this.size = size;
                    // TODO: not just pass through the given size, but rather the size that is necessary for scrollable content
                    try this.container.handle(.{ .resize = size });
                },
                .mouse => |mouse| {
                    std.log.debug("mouse event detected in scrollable element {any}", .{mouse.in(this.size)});
                    try this.container.handle(.{
                        .mouse = .{
                            .col = mouse.col + this.anchor.col,
                            .row = mouse.row + this.anchor.row,
                            .button = mouse.button,
                            .kind = mouse.kind,
                        },
                    });
                },
                else => try this.container.handle(event),
            }
        }

        fn content(ctx: *anyopaque, cells: []Cell, size: Size) !void {
            const this: *@This() = @ptrCast(@alignCast(ctx));
            std.debug.assert(cells.len == @as(usize, this.size.cols) * @as(usize, this.size.rows));

            const container_cells = try this.container.allocator.alloc(Cell, @as(usize, size.cols) * @as(usize, size.rows));
            @memset(cells, .{});
            {
                const container_cells_const = try this.container.contents();
                defer this.container.allocator.free(container_cells_const);
                const container_size = this.container.size;
                std.debug.assert(container_cells_const.len == @as(usize, container_size.cols) * @as(usize, container_size.rows));
                @memcpy(container_cells, container_cells_const);
            }

            for (this.container.elements.items) |*e| {
                const e_size = e.size;
                const element_cells = try e.contents();
                defer e.allocator.free(element_cells);

                const anchor = (@as(usize, e_size.anchor.row -| size.anchor.row) * @as(usize, size.cols)) + @as(usize, e_size.anchor.col -| size.anchor.col);
                var idx: usize = 0;
                blk: for (0..e_size.rows) |row| {
                    for (0..e_size.cols) |col| {
                        const cell = element_cells[idx];
                        idx += 1;

                        container_cells[anchor + (row * size.cols) + col].style = cell.style;
                        container_cells[anchor + (row * size.cols) + col].cp = cell.cp;

                        if (element_cells.len == idx) break :blk;
                    }
                }
            }

            const anchor = (@as(usize, this.anchor.row) * @as(usize, size.cols)) + @as(usize, this.anchor.col);
            for (container_cells, 0..) |cell, idx| {
                cells[anchor + idx] = cell;
            }
            this.container.allocator.free(container_cells);
        }
    };
}