#include <c_NonrecursiveDirectedDFS.h>
#include <c_ArrayStack.h>

typedef struct {
    c_size_t v;           /* Current vertex */
    c_size_t edge_idx;    /* Next neighbor index to examine in the adjacency list */
} c_DFSFrame_t;

c_err_t c_NonrecursiveDirectedDFS_Init(c_NonrecursiveDirectedDFS_t* self, c_Digraph_t* graph, c_size_t s, c_Allocator_t* allocator) {
    if (!self || !graph || s >= graph->V) return C_ERR_PARAM;

    self->allocator = allocator?*allocator:c_DefaultAllocator;
    self->count = 0;

    /* 1. Allocate the visited tracking array */
    self->marked = (c_bool_t*)c_Allocator_Calloc(&self->allocator, graph->V, sizeof(c_bool_t));
    if (!self->marked) return C_ERR_NOMEM;

    /* 2. Allocate an explicit execution stack bounded exactly by the number of vertices O(V) */
    c_DFSFrame_t frame={0};

    c_ArrayStack_t stack;
    c_err_t err = c_ArrayStack_Init(&stack, sizeof(c_DFSFrame_t), 0, allocator);
    if (err!=C_ERR_OK) {
        c_Allocator_Free(&self->allocator, self->marked);
        self->marked = NULL;
        return err;
    }

    /* Push the initial source vertex frame onto our stack */
    self->marked[s] = C_TRUE;
    self->count++;

    frame.v = s;
    frame.edge_idx = 0;
    c_ArrayStack_Push(&stack, &frame);

    /* 3. Non-recursive processing loop */
    while (!c_ArrayStack_IsEmpty(&stack)) {
        /* Peek the top frame */
        c_DFSFrame_t* current_frame = c_ArrayStack_Peek(&stack);

        c_size_t u = current_frame->v;

        c_AdjList_t* adj = &graph->adj_list[u];
        c_size_t neighbor_count = (c_size_t)c_AdjList_GetSize(adj);

        /* Advance down the adjacency list until we find an unvisited neighbor or finish the list */
        c_bool_t advanced = C_FALSE;
        while (current_frame->edge_idx < neighbor_count) {
            c_size_t w;
            c_AdjList_Get(adj, current_frame->edge_idx, &w);
            current_frame->edge_idx++; /* Increment pointer for next pass */

            if (!self->marked[w]) {
                self->marked[w] = C_TRUE;
                self->count++;

                /* Push new frame onto the stack (equivalent to recursive call invocation) */
                frame.v = w;
                frame.edge_idx = 0;
                c_ArrayStack_Push(&stack, &frame);

                advanced = C_TRUE;
                break;
            }
        }

        /* If we explored all neighbors of vertex u, pop it from the stack */
        if (!advanced) {
            c_ArrayStack_Pop(&stack, 0);
        }
    }

    /* Clean up the runtime frame stack buffer */
    c_ArrayStack_Destroy(&stack);
    return C_SUCCESS;
}

void c_NonrecursiveDirectedDFS_Destroy(c_NonrecursiveDirectedDFS_t* self) {
    if (!self) return;
    if (self->marked) {
        c_Allocator_Free(&self->allocator, self->marked);
        self->marked = NULL;
    }
    self->count = 0;
}