#include <c_Cycle.h>

/* ------------------------------------------------------------------------------------------------------------------ */
/*  */


// Internal deep structural trace walker looking for loops
static void c_Cycle_DFS_Internal(c_Cycle_t* self, const c_Graph_t* G, c_VertexId_t v, c_VertexId_t u) {
    self->marked[v] = C_TRUE;

    c_AdjList_t* list = c_Graph_GetAdjList((c_Graph_t*)G, v);
    if (!list) return;

    // Cache-friendly sequential sweep over flat neighbor array blocks
    for (c_size_t i = 0; i < list->size; i++) {
        const c_VertexId_t w = (c_VertexId_t)list->array[i];

        // Short-circuit search operations if a cycle has already been populated
        if (self->cycle.size > 0) return;

        if (!self->marked[w]) {
            self->edge_to[w] = v;
            c_Cycle_DFS_Internal(self, G, w, v);
        }
        // Undirected graph cycle criteria: w is visited AND w is not the direct parent u
        else if (w != u) {
            self->has_cycle = C_TRUE;

            // Reconstruct the cycle path back through the path logs
            c_VertexIdList_t temp_stack;
            if (c_VertexIdList_Init(&temp_stack, 8, &self->allocator) != C_SUCCESS) return;

            c_VertexId_t x = v;
            while (x != w && x != G->V) {
                c_VertexIdList_Append(&temp_stack, (c_uint_t)x);
                x = self->edge_to[x];
            }
            c_VertexIdList_Append(&temp_stack, (c_uint_t)w);
            c_VertexIdList_Append(&temp_stack, (c_uint_t)v); // Close cycle track frame

            // Invert elements to maintain chronological loop sequence (w -> ... -> v -> w)
            for (c_size_t j = temp_stack.size; j > 0; j--) {
                c_VertexId_t val;
                c_VertexIdList_Get(&temp_stack, j - 1, &val);
                c_VertexIdList_Append(&self->cycle, val);
            }

            c_VertexIdList_Destroy(&temp_stack);
            return;
        }
    }
}

c_err_t c_Cycle_Init(c_Cycle_t* self, const c_Graph_t* G, c_Allocator_t* allocator) {
    if (!self || !G ) return C_ERR_PARAM;

    self->allocator = allocator?*allocator:c_DefaultAllocator;
    self->has_cycle = C_FALSE;
    self->V = G->V;
    self->marked = NULL;
    self->edge_to = NULL;

    if (c_VertexIdList_Init(&self->cycle, 0, &self->allocator) != C_SUCCESS) {
        return C_ERR_NOMEM;
    }

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

    self->marked = (c_bool_t*)c_Allocator_Alloc(&self->allocator, G->V * sizeof(c_bool_t));
    self->edge_to = (c_size_t*)c_Allocator_Alloc(&self->allocator, G->V * sizeof(c_size_t));

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

    memset(self->marked, 0, G->V * sizeof(c_bool_t));
    for (c_size_t i = 0; i < G->V; i++) self->edge_to[i] = G->V; // Sentinel setup

    // Multi-component partition loop scans
    for (c_VertexId_t v = 0; v < G->V; v++) {
        if (!self->marked[v]) {
            c_Cycle_DFS_Internal(self, G, v, G->V); // Pass sentinel as initial parent
            if (self->has_cycle) break;
        }
    }

    return C_SUCCESS;
}

void c_Cycle_Destroy(c_Cycle_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);

    c_VertexIdList_Destroy(&self->cycle);

    self->marked = NULL;
    self->edge_to = NULL;
    self->has_cycle = C_FALSE;
    self->V = 0;
}

c_bool_t c_Cycle_HasCycle(const c_Cycle_t* self) {
    return self ? self->has_cycle : C_FALSE;
}

const c_VertexIdList_t* c_Cycle_Path(const c_Cycle_t* self) {
    return self ? &self->cycle : NULL;
}