#include <stdint.h>
#include <stdlib.h>
#include <assert.h>
#include <memory.h>

struct node_parent {
    char tag;
};

struct stack {
    int32_t size;
    int32_t count;
    struct node_parent** data;
};

struct node_app {
    char tag;
    struct node_parent* left;
    struct node_parent* right;
};

struct node_num {
    char tag;
    int32_t value;
};

struct node_global {
    char tag;
    int32_t arity;
    void (*function)(struct stack*);
};

struct node_ind {
    char tag;
    struct node_parent* next;
};

void stack_init(struct stack* stack) {
    stack->size = 16;
    stack->count = 0;
    stack->data = malloc(sizeof(*stack->data) * stack->size);
}

void stack_free(struct stack* stack) {
    free(stack->data);
}

void stack_push(struct stack* stack, struct node_parent* node) {
    assert(stack->count < stack->size);
    stack->data[stack->count++] = node;
}

struct node_parent* stack_peek(struct stack* stack, int32_t offset) {
    assert(offset + 1 <= stack->count);
    return stack->data[stack->count - offset - 1];
}

struct node_parent* stack_pop(struct stack* stack) {
    assert(stack->count > 0);
    return stack->data[--stack->count];
}

void stack_popn(struct stack* stack, int32_t count) {
    assert(stack->count >= count);
    stack->count -= count;
}

void stack_update(struct stack* stack, int32_t offset) {
    assert(stack->count >= offset + 2);
    struct node_ind* to_replace = (struct node_ind*) stack->data[stack->count - 1 - 1 - offset];
    to_replace->tag = 3;
    to_replace->next = stack->data[stack->count - 1];
    stack->count--;
}

void stack_alloc(struct stack* stack, int32_t count) {
    assert(stack->count + count <= stack->size);
    for(int32_t i = 0; i < count; i++) {
        stack->data[stack->count++] = malloc(sizeof(struct node_app));
    }
}

void stack_slide(struct stack* stack, int32_t count) {
    assert(stack->count > count); 
    stack->data[stack->count - 1 - count] = stack->data[stack->count - 1];
    stack->count -= count;
}

void stack_set(struct stack* stack, int32_t index, struct node_parent* parent) {
    assert(stack->count > index);
    stack->data[stack->count - 1 - index] = parent;
}

int32_t stack_size(struct stack* stack) {
    return stack->count;
}

struct node_parent* malloc_node_num(int32_t value) {
    struct node_num* node = malloc(sizeof(struct node_app));
    node->tag = 0;
    node->value = value;
    return (struct node_parent*) node;
}

struct node_parent* malloc_node_app(struct node_parent* left, struct node_parent* right) {
    struct node_app* node = malloc(sizeof(struct node_app));
    node->tag = 1;
    node->left = left;
    node->right = right;
    return (struct node_parent*) node;
}
struct node_parent* malloc_node_global(int32_t arity, void (*function)(struct stack*)) {
    struct node_global* node = malloc(sizeof(struct node_app));
    node->tag = 2;
    node->arity = arity;
    node->function = function;
    return (struct node_parent*) node;
}
struct node_parent* malloc_node_indirect(struct node_parent* target) {
    struct node_ind* node = malloc(sizeof(struct node_app));
    node->tag = 3;
    node->next = target;
    return (struct node_parent*) node;
}

void unwind(struct stack* stack) {
    while(1) {
        assert(stack_size(stack) != 0);
        struct node_parent* node = stack_peek(stack, 0);
        if(node->tag == 0) {
            return;
        } else if(node->tag == 1) {
            stack_push(stack, ((struct node_app*) node)->left);
        } else if(node->tag == 2) {
            struct node_global* global = (struct node_global*) node;
            if(stack->size > global->arity) {
                struct node_parent* root = stack_peek(stack, global->arity);
                stack_popn(stack, global->arity + 1);
                stack_push(stack, root);
                while(root->tag == 1) {
                    struct node_app* app = (struct node_app*) root;
                    stack_push(stack, app->right);
                    root = app->left;
                }
                global->function(stack);
            } else {
                stack_popn(stack, stack_size(stack) - 1);
                return;
            }
        } else if(node->tag == 3) {
            struct node_ind* ind = (struct node_ind*) node;
            stack_pop(stack);
            stack_push(stack, ind->next);
        }
    }
}

struct node_parent* eval(struct node_parent* start) {
    struct stack new_stack;
    stack_init(&new_stack);
    stack_push(&new_stack, start);
    unwind(&new_stack);
    struct node_parent* final_node = stack_pop(&new_stack);
    stack_free(&new_stack);
    return final_node;
}

extern void main_supercomb(struct stack* stack);

int main(int argc, char** argv) {
    struct node_parent* result = eval(malloc_node_global(0, main_supercomb));
}