#pragma once

#include <assert.h>
#include <string.h>

#define COROUTINE __attribute__((noreturn)) void

enum {
  COROUTINE_REGISTERS =
  19  /* 18 general purpose registers (r14-r31) and 1 return address */
  + 4  /* space for fiber_entry() to store the link register */
};

struct coroutine_context
{
    void **stack_pointer;
};

typedef COROUTINE(* coroutine_start)(struct coroutine_context *from, struct coroutine_context *self);

static inline void coroutine_initialize_main(struct coroutine_context * context) {
    context->stack_pointer = NULL;
}

static inline void coroutine_initialize(
    struct coroutine_context *context,
    coroutine_start start,
    void *stack,
    size_t size
) {
    assert(start && stack && size >= 1024);

    // Stack grows down. Force 16-byte alignment.
    char * top = (char*)stack + size;
    context->stack_pointer = (void**)((uintptr_t)top & ~0xF);

    context->stack_pointer -= COROUTINE_REGISTERS;
    memset(context->stack_pointer, 0, sizeof(void*) * COROUTINE_REGISTERS);

    /* Skip a global prologue that sets the TOC register */
    context->stack_pointer[18] = ((char*)start) + 8;
}

struct coroutine_context * coroutine_transfer(struct coroutine_context * current, struct coroutine_context * target);

static inline void coroutine_destroy(struct coroutine_context * context)
{
    context->stack_pointer = NULL;
}