enumerate-balanced-ideals/thickenings.c

#define _GNU_SOURCE

#include <stdio.h>
#include <limits.h>
#include <stdlib.h>
#include <malloc.h>
#include <memory.h>

#include "thickenings.h"
#include "coxeter.h"
#include "queue.h"

char *alphabetize(int *word, int len, const char *alphabet, char *buffer)
{
  int i = 0;
  for(i = 0; i < len; i++)
    buffer[i] = alphabet[word[i]];
  buffer[i] = 0;

  return buffer;
}

void print_balanced_thickening(int rank, int order, const int *thickening, const int *left_invariant, const int *right_invariant, const char *alphabet, FILE *f)
{
  for(int i = 0; i < order; i++) {
    if(thickening[i])
      fprintf(f, "x");
    else
      fprintf(f, "0");
  }

  fprintf(f, " left: ");
  for(int j = 0; j < rank; j++)
    if(left_invariant[j])
      fprintf(f, "%c", alphabet[j]);
    else
      fprintf(f, " ");

  fprintf(f, " right: ");
  for(int j = 0; j < rank; j++)
    if(right_invariant[j])
      fprintf(f, "%c", alphabet[j]);
    else
      fprintf(f, " ");

  fprintf(f, "\n");
}

static int compare_wordlength(const void *a, const void *b, void *gr)
{
  int i = *((int*)a);
  int j = *((int*)b);
  node_t *graph = (node_t*)gr;

  return graph[i].wordlength - graph[j].wordlength;
}

void prepare_graph(semisimple_type_t type, node_t *graph, edgelist_t **edgelists_pointer, int **words_pointer) // the edgelists_pointer and words_pointer arguments are just for freeing afterwards
{
  queue_t queue;

  int rank, order;
  edgelist_t *edge, *previous;
  int edgelist_count, max_wordlength, hyperplane_count;
  int current;

  int *graph_data;
  node_t *graph_unsorted;
  int *wordlength_order, *reverse_wordlength_order, *seen, *words;
  edgelist_t *edgelists;

  // initialize

  rank = coxeter_rank(type);
  order = coxeter_order(type);

  graph_data = (int*)malloc(order*rank*sizeof(int));
  graph_unsorted = (node_t*)malloc(order*sizeof(node_t));
  wordlength_order = (int*)malloc(order*sizeof(int));
  reverse_wordlength_order = (int*)malloc(order*sizeof(int));
  seen = (int*)malloc(order*sizeof(int));

  for(int i = 0; i < order; i++) {
    graph_unsorted[i].left = graph[i].left;
    graph_unsorted[i].right = graph[i].right;
    graph_unsorted[i].word = 0;
    graph_unsorted[i].wordlength = INT_MAX;
    graph_unsorted[i].bruhat_lower = 0;
    graph_unsorted[i].bruhat_higher = 0;
    graph_unsorted[i].is_hyperplane_reflection = 0;
  }

  // get coxeter graph

  generate_coxeter_graph(type, graph_data);

  for(int i = 0; i < order; i++)
    for(int j = 0; j < rank; j++)
      graph_unsorted[i].left[j] = graph_data[i*rank + j];

  // find wordlengths

  graph_unsorted[0].wordlength = 0;
  queue_init(&queue);
  queue_put(&queue, 0);
  while((current = queue_get(&queue)) != -1) {
    for(int i = 0; i < rank; i++) {
      int neighbor = graph_unsorted[current].left[i];
      if(graph_unsorted[neighbor].wordlength > graph_unsorted[current].wordlength + 1) {
	graph_unsorted[neighbor].wordlength = graph_unsorted[current].wordlength + 1;
	queue_put(&queue, neighbor);
      }
    }
  }

  max_wordlength = 0;
  for(int i = 0; i < order; i++)
    if(graph_unsorted[i].wordlength > max_wordlength)
      max_wordlength = graph_unsorted[i].wordlength;

  // sort by wordlength

  for(int i = 0; i < order; i++)
    wordlength_order[i] = i;
  qsort_r(wordlength_order, order, sizeof(int), compare_wordlength, graph_unsorted); // so wordlength_order is a map new index -> old index
  for(int i = 0; i < order; i++)
    reverse_wordlength_order[wordlength_order[i]] = i; // reverse_wordlength_order is a map old index -> new index
  for(int i = 0; i < order; i++) {
    graph[i] = graph_unsorted[wordlength_order[i]]; // copy the whole thing
    for(int j = 0; j < rank; j++)
      graph[i].left[j] = reverse_wordlength_order[graph[i].left[j]]; // rewrite references
  }

  // find words

  words = (int*)malloc(order*max_wordlength*sizeof(int));
  memset(words, 0, order*max_wordlength*sizeof(int));
  graph[0].word = &words[0];
  queue_init(&queue);
  queue_put(&queue, 0);
  while((current = queue_get(&queue)) != -1) {
    for(int i = 0; i < rank; i++) {
      int neighbor = graph[current].left[i];
      if(graph[neighbor].wordlength == graph[current].wordlength + 1 && graph[neighbor].word == 0) {
	graph[neighbor].word = &words[neighbor*max_wordlength];
	memcpy(&graph[neighbor].word[1], &graph[current].word[0], graph[current].wordlength*sizeof(int));
	graph[neighbor].word[0] = i;
	queue_put(&queue, neighbor);
      }
    }
  }

  // generate right edges

  for(int i = 0; i < order; i++) {
    for(int j = 0; j < rank; j++) {
      current = graph[0].left[j];
      for(int k = graph[i].wordlength - 1; k >= 0; k--) { // apply group element from right to left
	current = graph[current].left[graph[i].word[k]];
      }
      graph[i].right[j] = current;
    }
  }

  // find opposites

  node_t *longest = &graph[order-1];
  for(int i = 0; i < order; i++) {
    current = i;
    for(int k = longest->wordlength - 1; k >= 0; k--)
      current = graph[current].left[longest->word[k]];
    graph[i].opposite = current;
  }

  // enumerate hyperplanes

  hyperplane_count = 0;
  for(int i = 0; i < order; i++) {
    for(int j = 0; j < rank; j++) {
      current = 0;
      int *word1 = graph[i].word;
      int word1len = graph[i].wordlength;
      int *word2 = graph[graph[i].right[j]].word; // want to calculate word2 * word1^{-1}
      int word2len = graph[graph[i].right[j]].wordlength;
      for(int k = 0; k < word1len; k++) // apply inverse, i.e. go from left to right
	current = graph[current].left[word1[k]];
      for(int k = word2len - 1; k >= 0; k--) // now from right to left
	current = graph[current].left[word2[k]];
      if(graph[current].is_hyperplane_reflection == 0) {
	graph[current].is_hyperplane_reflection = 1;
	hyperplane_count++;
      }
    }
  }

  // generate folding order

  edgelists = (edgelist_t*)malloc(order*hyperplane_count*sizeof(edgelist_t));
  edgelist_count = 0;
  for(int i = 0; i < order; i++) {
    if(graph[i].is_hyperplane_reflection) {
      for(int j = 0; j < order; j++) {

	current = j;
	for(int k = graph[i].wordlength - 1; k >= 0; k--) // apply hyperplane reflection
	  current = graph[current].left[graph[i].word[k]];

	if(graph[j].wordlength < graph[current].wordlength) { // current has higher bruhat order than j
	  edgelists[edgelist_count].to = j;
	  edgelists[edgelist_count].next = graph[current].bruhat_lower;
	  graph[current].bruhat_lower = &edgelists[edgelist_count];
	  edgelist_count++;
	} else if(graph[j].wordlength > graph[current].wordlength) { // j has higher bruhat order than current; these are already included from the other side
	} else {
	  ERROR(1, "Chambers of equal word lengths should not be folded on each other!\n");
	}
      }
    }
  }

  // remove redundant edges

  for(int i = 0; i < order; i++) {
    memset(seen, 0, order*sizeof(int));
    for(int len = 1; len <= max_wordlength; len++) {
      // remove all edges originating from i of length len which connect to something already seen using shorter edges
      edge = graph[i].bruhat_lower;
      previous = (edgelist_t*)0;
      while(edge) {
	if(seen[edge->to] && graph[i].wordlength - graph[edge->to].wordlength == len) {
	  //	  fprintf(stderr, "deleting from %d to %d\n", i, edge->to);
	  if(previous)
	    previous->next = edge->next;
	  else
	    graph[i].bruhat_lower = edge->next;
	} else {
	  previous = edge;
	}
	edge = edge->next;
      }

      // see which nodes we can reach using only edges up to length len, mark them as seen
      queue_init(&queue);
      queue_put(&queue, i);
      seen[i] = 1;
      while((current = queue_get(&queue)) != -1) {
	edge = graph[current].bruhat_lower;
	while(edge) {
	  if(!seen[edge->to] && graph[current].wordlength - graph[edge->to].wordlength == len) {
	    seen[edge->to] = 1;
	    queue_put(&queue, edge->to);
	  }
	  edge = edge->next;
	}
      }
    }
  }

  // reverse folding order

  for(int i = 0; i < order; i++) {
    edge = graph[i].bruhat_lower;
    while(edge) {
      edgelists[edgelist_count].to = i;
      edgelists[edgelist_count].next = graph[edge->to].bruhat_higher;
      graph[edge->to].bruhat_higher = &edgelists[edgelist_count];
      edgelist_count++;
      edge = edge->next;
    }
  }

  *edgelists_pointer = edgelists;
  *words_pointer = words;

  free(graph_data);
  free(graph_unsorted);
  free(wordlength_order);
  free(reverse_wordlength_order);
  free(seen);
}

void enumerate_balanced_thickenings(semisimple_type_t type, node_t *graph, const char *alphabet, FILE *outfile)
{
  int rank, order;
  int *level;
  int *left_invariant, *right_invariant;
  long thickenings_count, fat_count, slim_count, balanced_count;
  int is_fat, is_slim;
  int current_level, head, current;
  int i;
  edgelist_t *edge;

  queue_t queue;

  rank = coxeter_rank(type);
  order = coxeter_order(type);

  level = (int*)malloc(order*sizeof(int));
  left_invariant = (int*)malloc(rank*sizeof(int));
  right_invariant = (int*)malloc(rank*sizeof(int));

  thickenings_count = fat_count = slim_count = balanced_count = 0;
  memset(level, 0, order*sizeof(int));
  current_level = 1;
  head = order - 1;
  level[head] = -1;
  while(current_level > 0) {
    // calculate transitive closure
    queue_init(&queue);
    queue_put(&queue, head);
    while((current = queue_get(&queue)) != -1) {
      edge = graph[current].bruhat_lower;
      while(edge) {
	if(level[edge->to] == 0) {
	  level[edge->to] = current_level;
	  queue_put(&queue, edge->to);
	}
	edge = edge->next;
      }
    }

    // we have a thickening, do something with it!

    is_fat = is_slim = 1;
    for(int i = 0; i < order; i++) {
      if(level[graph[i].opposite] != 0) {
	if(level[i] != 0)
	  is_slim = 0;
      } else {
	if(level[i] == 0)
	  is_fat = 0;
      }
    }

    // count
    thickenings_count++;
    if(is_fat)
      fat_count++;
    if(is_slim)
      slim_count++;
    if(is_slim && is_fat) {
      ERROR(balanced_count >= MAX_THICKENINGS, "Too many balanced thickenings! Increase MAX_THICKENINGS\n");
      //memcpy(&balanced_thickenings[balanced_count*order], level, order*sizeof(int));
      fwrite(level, sizeof(int), order, outfile);
      balanced_count++;
    }

    // print out the thickening
    if(is_fat && is_slim) {
      // check for invariances
      for(int j = 0; j < rank; j++) {
    	left_invariant[j] = 1;
    	right_invariant[j] = 1;
      }
      for(int i = 0; i < order; i++) {
    	for(int j = 0; j < rank; j++) {
    	  if(level[i] == 0 && level[graph[i].left[j]] != 0 || level[i] != 0 && level[graph[i].left[j]] == 0)
    	    left_invariant[j] = 0;
    	  if(level[i] == 0 && level[graph[i].right[j]] != 0 || level[i] != 0 && level[graph[i].right[j]] == 0)
    	    right_invariant[j] = 0;
    	}
      }
      print_balanced_thickening(rank, order, level, left_invariant, right_invariant, alphabet, stderr);
    }


    // now find the next one!

    // try to find empty spot to the left of "head"
    for(i = head - 1; i >= 0; i--)
      if(level[i] == 0)
    	break;
    if(i >= 0) {
      head = i;
      level[head] = -1;
      current_level++;
      continue;
    }

    // if none was found, try to move "head" to the left
    while(current_level > 0) {
      for(i = head - 1; i >= 0; i--)
	if(level[i] == 0 || level[i] >= current_level)
	  break;
      if(i >= 0) { // if this was successful, just move head
	level[head] = 0;
	head = i;
	level[head] = -1;
	break;
      } else { // if moving the head is not possible, take the next head to the right
	current_level--;
	level[head] = 0;
	do {
	  head++;
	} while(head < order && level[head] != -1);
      }
    }

    // clean up
    for(int i = 0; i < head; i++)
      if(level[i] >= current_level)
	level[i] = 0;
  }

  fprintf(stderr, "\n");
  fprintf(stderr, "Found %ld thickenings, %ld fat, %ld slim, %ld balanced\n\n", thickenings_count, fat_count, slim_count, balanced_count);

  free(level);
  free(left_invariant);
  free(right_invariant);
}