const std = @import("std");
const stdx = @import("stdx");
const builtin = @import("builtin");
const Error = @import("coro.zig").Error;

const ArchInfo = struct {
    num_registers: usize,
    jump_idx: usize,
    assembly: []const u8,
};

const arch_info: ArchInfo = switch (builtin.cpu.arch) {
    .aarch64 => .{
        .num_registers = 20,
        .jump_idx = 19,
        .assembly = @embedFile("asm/coro_aarch64.s"),
    },
    .x86_64 => switch (builtin.os.tag) {
        .windows => .{
            .num_registers = 32,
            .jump_idx = 30,
            .assembly = @embedFile("asm/coro_x86_64_windows.s"),
        },
        else => .{
            .num_registers = 8,
            .jump_idx = 6,
            .assembly = @embedFile("asm/coro_x86_64.s"),
        },
    },
    .riscv64 => .{
        .num_registers = 25,
        .jump_idx = 24,
        .assembly = @embedFile("asm/coro_riscv64.s"),
    },
    else => @compileError("Unsupported cpu architecture"),
};

pub const stack_alignment = 16;

extern fn libcoro_stack_swap(current: *Coro, target: *Coro) void;
comptime {
    asm (arch_info.assembly);
}

pub const Coro = packed struct {
    stack_pointer: [*]u8,

    const Self = @This();
    const Func = *const fn (
        from: *Coro,
        self: *Coro,
    ) callconv(.C) noreturn;

    pub fn init(func: Func, stack: []align(stack_alignment) u8) !Self {
        stdx.debug.assertComptime(@sizeOf(usize) == 8, "usize expected to take 8 bytes", .{});
        stdx.debug.assertComptime(@sizeOf(*Func) == 8, "function pointer expected to take 8 bytes", .{});

        const register_bytes = arch_info.num_registers * 8;
        if (register_bytes > stack.len) {
            return Error.StackTooSmall;
        }
        const register_space = stack[stack.len - register_bytes ..];
        const jump_ptr: *Func = @ptrCast(@alignCast(&register_space[arch_info.jump_idx * 8]));
        jump_ptr.* = func;
        return .{ .stack_pointer = register_space.ptr };
    }

    pub inline fn resumeFrom(self: *Self, from: *Self) void {
        libcoro_stack_swap(from, self);
    }
};