cKit/Graph/c_NonrecursiveTarjanSCC.c

#include <c_NonrecursiveTarjanSCC.h>
#include <c_Macros.h>

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

/* Private iterative DFS engine subroutine */
static void c_NonrecursiveTarjanSCC_Process(c_NonrecursiveTarjanSCC_t* self, c_Digraph_t* graph, c_size_t root, c_TarjanFrame_t* frame_stack) {
    c_size_t frame_stack_size = 0;

    /* Push the initial root activation frame entry */
    self->marked[root] = C_TRUE;
    self->pre[root] = self->pre_counter++;
    self->low[root] = self->pre[root];
    self->stack[self->stack_size++] = root;
    self->on_stack[root] = C_TRUE;

    frame_stack[frame_stack_size++] = (c_TarjanFrame_t){ .v = root, .edge_idx = 0 };

    while (frame_stack_size > 0) {
        c_TarjanFrame_t* current_frame = &frame_stack[frame_stack_size - 1];
        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);

        c_bool_t advanced = C_FALSE;
        while (current_frame->edge_idx < neighbor_count) {
            c_uint_t target_value = 0;
            c_err_t err = c_AdjList_Get(adj, current_frame->edge_idx, &target_value);
            current_frame->edge_idx++; /* Advance edge iterator pointer state */

            if (err == C_SUCCESS) {
                c_size_t w = (c_size_t)target_value;

                if (!self->marked[w]) {
                    self->marked[w] = C_TRUE;
                    self->pre[w] = self->pre_counter++;
                    self->low[w] = self->pre[w];
                    self->stack[self->stack_size++] = w;
                    self->on_stack[w] = C_TRUE;

                    /* Push new execution frame onto the state stack (Simulating Recursive Call) */
                    frame_stack[frame_stack_size++] = (c_TarjanFrame_t){ .v = w, .edge_idx = 0 };
                    advanced = C_TRUE;
                    break;
                }
                else if (self->on_stack[w]) {
                    self->low[u] = C_MIN(self->low[u], self->pre[w]);
                }
            }
        }

        if (advanced) continue; /* Yield control forward down into the child node branch */

        /* Post-visit Processing (Equivalent to tracking back up the recursive unwinding stack) */
        frame_stack_size--; /* Pop frame context */

        if (frame_stack_size > 0) {
            c_size_t parent = frame_stack[frame_stack_size - 1].v;
            self->low[parent] = C_MIN(self->low[parent], self->low[u]);
        }

        /* If u is a root node of an SCC, pop all component nodes off the membership tracker */
        if (self->low[u] == self->pre[u]) {
            c_size_t component_node = 0;
            do {
                component_node = self->stack[--self->stack_size];
                self->id[component_node] = self->count;
                self->on_stack[component_node] = C_FALSE;
            } while (component_node != u);

            self->count++; /* Advance component group numbering identity mapping */
        }
    }
}

c_err_t c_NonrecursiveTarjanSCC_Init(c_NonrecursiveTarjanSCC_t* self, c_Digraph_t* graph, c_Allocator_t* allocator) {
    if (!self || !graph) return C_ERR_PARAM;

    self->allocator = allocator ? *allocator : c_DefaultAllocator;
    self->count = 0;
    self->pre_counter = 0;
    self->stack_size = 0;
    self->V = graph->V;

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

    self->marked   = (c_bool_t*)c_Allocator_Calloc(&self->allocator, self->V, sizeof(c_bool_t));
    self->id       = (c_size_t*)c_Allocator_Calloc(&self->allocator, self->V, sizeof(c_size_t));
    self->pre      = (c_size_t*)c_Allocator_Calloc(&self->allocator, self->V, sizeof(c_size_t));
    self->low      = (c_size_t*)c_Allocator_Calloc(&self->allocator, self->V, sizeof(c_size_t));
    self->stack    = (c_size_t*)c_Allocator_Calloc(&self->allocator, self->V, sizeof(c_size_t));
    self->on_stack = (c_bool_t*)c_Allocator_Calloc(&self->allocator, self->V, sizeof(c_bool_t));

    if (!self->marked || !self->id || !self->pre || !self->low || !self->stack || !self->on_stack) {
        c_NonrecursiveTarjanSCC_Destroy(self);
        return C_ERR_NOMEM;
    }

    /* Allocate local frame execution engine tracking context sized O(V) */
    c_TarjanFrame_t* frame_stack = (c_TarjanFrame_t*)c_Allocator_Calloc(&self->allocator, self->V, sizeof(c_TarjanFrame_t));
    if (!frame_stack) {
        c_NonrecursiveTarjanSCC_Destroy(self);
        return C_ERR_NOMEM;
    }

    for (c_size_t v = 0; v < self->V; ++v) {
        if (!self->marked[v]) {
            c_NonrecursiveTarjanSCC_Process(self, graph, v, frame_stack);
        }
    }

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

void c_NonrecursiveTarjanSCC_Destroy(c_NonrecursiveTarjanSCC_t* self) {
    if (!self) return;
    if (self->marked)   c_Allocator_Free(&self->allocator, self->marked);
    if (self->id)       c_Allocator_Free(&self->allocator, self->id);
    if (self->pre)      c_Allocator_Free(&self->allocator, self->pre);
    if (self->low)      c_Allocator_Free(&self->allocator, self->low);
    if (self->stack)    c_Allocator_Free(&self->allocator, self->stack);
    if (self->on_stack) c_Allocator_Free(&self->allocator, self->on_stack);

    self->marked = NULL;
    self->id = NULL;
    self->pre = NULL;
    self->low = NULL;
    self->stack = NULL;
    self->on_stack = NULL;
    self->count = 0;
    self->stack_size = 0;
    self->V = 0;
}