#include <c_DirectedEulerianCycle.h>
#include "c_NonrecursiveDirectedDFS.h"

c_err_t c_DirectedEulerianCycle_Init(c_DirectedEulerianCycle_t* self, c_Digraph_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);

    /* Edge case: An empty graph with no edges trivially has an empty Eulerian cycle path */
    if (graph->E == 0) return C_SUCCESS;

    /* 1. Necessary condition verification: In-degree must equal Out-degree for all nodes */
    c_size_t non_isolated_vertex = graph->V;
    for (c_size_t v = 0; v < graph->V; ++v) {
        c_size_t out_deg = c_Digraph_GetOutDegree(graph, v);
        c_size_t in_deg = c_Digraph_GetInDegree(graph, v);

        if (out_deg != in_deg) {
            return C_SUCCESS; /* No Eulerian cycle exists, return gracefully with empty cycle path */
        }
        if (out_deg > 0 && non_isolated_vertex == graph->V) {
            non_isolated_vertex = v; /* Track a starting node candidates containing edges */
        }
    }

    if (non_isolated_vertex == graph->V) return C_SUCCESS; // Graph has no edges

    /* 2. Necessary condition verification: Connectivity check via non-recursive DFS */
    c_NonrecursiveDirectedDFS_t dfs;
    c_err_t err = c_NonrecursiveDirectedDFS_Init(&dfs, graph, non_isolated_vertex, allocator);
    if (err != C_SUCCESS) return err;

    for (c_size_t v = 0; v < graph->V; ++v) {
        if (c_Digraph_GetOutDegree(graph, v) > 0 && !c_NonrecursiveDirectedDFS_HasPathTo(&dfs, v, graph->V)) {
            c_NonrecursiveDirectedDFS_Destroy(&dfs);
            return C_SUCCESS; /* Graph edges are disconnected */
        }
    }
    c_NonrecursiveDirectedDFS_Destroy(&dfs);

    /* 3. Hierholzer's Algorithm: Construct the cycle path */
    /* Maintain local tracking copies of current edge indexing to safely 'consume' them */
    c_size_t* edge_index_iterator = (c_size_t*)c_Allocator_Calloc(&self->allocator, graph->V, sizeof(c_size_t));
    if (!edge_index_iterator) return C_ERR_NOMEM;

    /* Stack setup for path assembly tracking: capacity bounded by E + 1 */
    c_size_t max_stack_cap = graph->E + 1;
    c_size_t* path_stack = (c_size_t*)c_Allocator_Calloc(&self->allocator, max_stack_cap, sizeof(c_size_t));
    if (!path_stack) {
        c_Allocator_Free(&self->allocator, edge_index_iterator);
        return C_ERR_NOMEM;
    }

    /* Temporary collector stack tracking for final inversion layout */
    c_size_t* output_stack = (c_size_t*)c_Allocator_Calloc(&self->allocator, max_stack_cap, sizeof(c_size_t));
    if (!output_stack) {
        c_Allocator_Free(&self->allocator, edge_index_iterator);
        c_Allocator_Free(&self->allocator, path_stack);
        return C_ERR_NOMEM;
    }

    c_size_t path_stack_size = 0;
    c_size_t output_stack_size = 0;

    /* Start the path construction from our candidate node */
    path_stack[path_stack_size++] = non_isolated_vertex;

    while (path_stack_size > 0) {
        c_size_t curr_v = path_stack[path_stack_size - 1];
        c_AdjList_t* adj = &graph->adj_list[curr_v];
        c_size_t out_degree = (c_size_t)c_UIntArray_GetSize(adj);

        if (edge_index_iterator[curr_v] < out_degree) {
            c_uint_t target_w = 0;
            err = c_UIntArray_Get(adj, edge_index_iterator[curr_v], &target_w);
            edge_index_iterator[curr_v]++; /* Consume the edge */

            if (err == C_SUCCESS) {
                path_stack[path_stack_size++] = (c_size_t)target_w;
            }
        } else {
            /* Backtrack step: Node has no remaining active edges out, push to tour results */
            output_stack[output_stack_size++] = curr_v;
            path_stack_size--;
        }
    }

    /* Push collected route into the formal c_VertexIdList storage layout matching target sequential format */
    while (output_stack_size > 0) {
        c_VertexIdList_Append(&self->cycle, (c_uint_t)output_stack[--output_stack_size]);
    }

    /* Clean up local working variables */
    c_Allocator_Free(&self->allocator, edge_index_iterator);
    c_Allocator_Free(&self->allocator, path_stack);
    c_Allocator_Free(&self->allocator, output_stack);

    return C_SUCCESS;
}

void c_DirectedEulerianCycle_Destroy(c_DirectedEulerianCycle_t* self) {
    if (!self) return;
    c_VertexIdList_Destroy(&self->cycle);
}

c_err_t c_DirectedEulerianCycle_GetCycle(c_DirectedEulerianCycle_t* self, c_VertexIdList_t* out_cycle) {
    if (!self || !out_cycle) return C_ERR_PARAM;
    if (!c_DirectedEulerianCycle_HasCycle(self)) return C_ERR_FAIL;

    c_size_t size = (c_size_t)c_VertexIdList_GetSize(&self->cycle);
    for (c_size_t i = 0; i < size; ++i) {
        c_uint_t val = 0;
        c_err_t err = c_VertexIdList_Get((c_VertexIdList_t*)&self->cycle, i, &val);
        if (err == C_SUCCESS) {
            c_err_t app_err = c_VertexIdList_Append(out_cycle, val);
            if (app_err != C_SUCCESS) return app_err;
        }
    }
    return C_SUCCESS;
}