#include <c_EdgeWeightedDirectedCycle.h>


#define CYCLE_SENTINEL ((c_size_t)-1)

typedef struct {
    c_size_t v;           /* Current vertex index */
    c_size_t edge_idx;    /* Next neighbor index to pull from adjacency out-list */
} c_DigraphCycleFrame_t;

/* Private non-recursive DFS machine loop processor */
static void c_EdgeWeightedDirectedCycle_Process(c_EdgeWeightedDirectedCycle_t* self, c_EdgeWeightedDigraph_t* graph, c_size_t root, c_DigraphCycleFrame_t* frame_stack) {
    c_size_t stack_size = 0;

    /* Push initial root activation block onto structural runtime stack */
    self->marked[root] = C_TRUE;
    self->on_stack[root] = C_TRUE;
    frame_stack[stack_size++] = (c_DigraphCycleFrame_t){ .v = root, .edge_idx = 0 };

    while (stack_size > 0) {
        c_DigraphCycleFrame_t* current_frame = &frame_stack[stack_size - 1];
        c_size_t u = current_frame->v;

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

        c_bool_t advanced = C_FALSE;
        while (current_frame->edge_idx < neighbor_count) {
            c_uint_t generic_edge_id = 0;
            c_err_t err = c_UIntArray_Get(adj, current_frame->edge_idx, &generic_edge_id);
            current_frame->edge_idx++; /* Advance out-list pointer position */

            if (err == C_SUCCESS) {
                c_size_t edge_id = (c_size_t)generic_edge_id;
                c_size_t w = graph->edges_pool[edge_id].to;

                /* Case A: Encountered an unvisited node, simulate recursive call descent */
                if (!self->marked[w]) {
                    self->marked[w] = C_TRUE;
                    self->on_stack[w] = C_TRUE;
                    self->edge_to[w] = edge_id;

                    frame_stack[stack_size++] = (c_DigraphCycleFrame_t){ .v = w, .edge_idx = 0 };
                    advanced = C_TRUE;
                    break;
                }
                /* Case B: Backedge detected (target node is currently alive on stack) -> Cycle Trapped! */
                else if (self->on_stack[w]) {
                    c_size_t* reverse_stack = (c_size_t*)c_Allocator_Calloc(&self->allocator, graph->V, sizeof(c_size_t));
                    if (!reverse_stack) return;

                    c_size_t trace_size = 0;
                    reverse_stack[trace_size++] = edge_id; /* Seal final back edge closing the loop */

                    /* Trace backward using edge origin nodes until reaching intersection w */
                    c_size_t curr_v = u;
                    while (curr_v != w) {
                        c_size_t prev_edge_id = self->edge_to[curr_v];
                        if (prev_edge_id == CYCLE_SENTINEL) break;

                        reverse_stack[trace_size++] = prev_edge_id;
                        curr_v = graph->edges_pool[prev_edge_id].from;
                    }

                    /* Append elements forward into persistent container matching execution path */
                    while (trace_size > 0) {
                        c_VertexIdList_Append(&self->cycle, (c_uint_t)reverse_stack[--trace_size]);
                    }

                    c_Allocator_Free(&self->allocator, reverse_stack);
                    return; /* Return instantly to stop unnecessary processing */
                }
            }
        }

        /* Short-circuit bubbling up if an evaluation path already verified a cycle */
        if (c_EdgeWeightedDirectedCycle_HasCycle(self)) return;

        /* If all outbound avenues from node u are fully parsed, pop it from execution tracking */
        if (!advanced) {
            self->on_stack[u] = C_FALSE;
            stack_size--;
        }
    }
}

c_err_t c_EdgeWeightedDirectedCycle_Init(c_EdgeWeightedDirectedCycle_t* self, c_EdgeWeightedDigraph_t* graph, c_Allocator_t* allocator) {
    if (!self || !graph) return C_ERR_PARAM;

    self->allocator = allocator ? *allocator : c_DefaultAllocator;
    c_VertexIdList_Init(&self->cycle, 0, allocator);

    if (graph->V == 0) return C_SUCCESS;

    self->marked   = (c_bool_t*)c_Allocator_Calloc(&self->allocator, graph->V, sizeof(c_bool_t));
    self->edge_to  = (c_size_t*)c_Allocator_Calloc(&self->allocator, graph->V, sizeof(c_size_t));
    self->on_stack = (c_bool_t*)c_Allocator_Calloc(&self->allocator, graph->V, sizeof(c_bool_t));

    if (!self->marked || !self->edge_to || !self->on_stack) {
        c_EdgeWeightedDirectedCycle_Destroy(self);
        return C_ERR_NOMEM;
    }

    for (c_size_t i = 0; i < graph->V; ++i) {
        self->edge_to[i] = CYCLE_SENTINEL;
    }

    /* Allocate continuous explicit compiler-emulated frame scratch buffer stack O(V) */
    c_DigraphCycleFrame_t* frame_stack = (c_DigraphCycleFrame_t*)c_Allocator_Calloc(&self->allocator, graph->V, sizeof(c_DigraphCycleFrame_t));
    if (!frame_stack) {
        c_EdgeWeightedDirectedCycle_Destroy(self);
        return C_ERR_NOMEM;
    }

    /* Sweep disconnected structural pockets across graph nodes boundary safely */
    for (c_size_t v = 0; v < graph->V; ++v) {
        if (!self->marked[v] && !c_EdgeWeightedDirectedCycle_HasCycle(self)) {
            c_EdgeWeightedDirectedCycle_Process(self, graph, v, frame_stack);
        }
    }

    c_Allocator_Free(&self->allocator, frame_stack);
    return C_SUCCESS;
}

void c_EdgeWeightedDirectedCycle_Destroy(c_EdgeWeightedDirectedCycle_t* self) {
    if (!self) return;
    if (self->marked)   c_Allocator_Free(&self->allocator, self->marked);
    if (self->edge_to)  c_Allocator_Free(&self->allocator, self->edge_to);
    if (self->on_stack) c_Allocator_Free(&self->allocator, self->on_stack);

    c_VertexIdList_Destroy(&self->cycle);

    self->marked = NULL;
    self->edge_to = NULL;
    self->on_stack = NULL;
}

c_err_t c_EdgeWeightedDirectedCycle_GetCycle(c_EdgeWeightedDirectedCycle_t* self, c_VertexIdList_t* out_cycle) {
    if (!self || !out_cycle) return C_ERR_PARAM;
    if (!c_EdgeWeightedDirectedCycle_HasCycle(self)) return C_ERR_FAIL;

    /* Copy pre-computed internal cycle sequence using high-speed block layout rules */
    return c_UIntArray_Copy(out_cycle, (c_UIntArray_t*)&self->cycle);
}