From ee3cceeb7a4428f0200959b3036ef26f28a16e91 Mon Sep 17 00:00:00 2001
From: Rob Landley <rob@landley.net>
Date: Mon, 11 Sep 2023 08:00:51 -0500
Subject: [PATCH] First stab at tsort.

---
 toys/pending/tsort.c | 95 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 95 insertions(+)
 create mode 100644 toys/pending/tsort.c

diff --git a/toys/pending/tsort.c b/toys/pending/tsort.c
new file mode 100644
index 00000000..20dd99b2
--- /dev/null
+++ b/toys/pending/tsort.c
@@ -0,0 +1,95 @@
+/* tsort.c - topological sort
+ *
+ * Copyright 2023 Rob Landley <rob@landley.net>
+ *
+ * See http://opengroup.org/onlinepubs/9699919799/utilities/tsort.html
+
+USE_TSORT(NEWTOY(tsort, ">1", TOYFLAG_USR|TOYFLAG_BIN))
+
+config TSORT
+  bool "tsort"
+  default n
+  help
+    usage: tsort [FILE]
+
+    Topological sort: read pairs of input strings indicating before/after
+    relationships. Output sorted result if no cycles, or first cycle to stderr.
+*/
+
+#include "toys.h"
+
+// sort by second element
+static int klatch(char **a, char **b)
+{
+  return strcmp(a[1], b[1]);
+}
+
+// TODO: this treats NUL in input as EOF
+static void do_hersheba(int fd, char *name)
+{
+  off_t plen;
+  char *djel = readfd(fd, 0, &plen), *ss, **pair, *keep[2];
+  long ii, jj, kk, ll, len = 0, first = 1;
+
+  // Count input entries
+  if (!djel) return;
+  for (ss = djel;;) {
+    while (isspace(*ss)) ss++;
+    if (!*ss) break;
+    len++;
+    while (*ss && !isspace(*ss)) ss++;
+  }
+  if (len&1) error_exit("bad input (not pairs)");
+
+  // collect entries and null terminate strings
+  pair = xmalloc(len*sizeof(char *));
+  for (ss = djel, len = 0;;) {
+    while (isspace(*ss)) *ss++ = 0;
+    if (!*ss) break;
+    pair[len++] = ss;
+    while (*ss && !isspace(*ss)) ss++;
+  }
+
+  // sort by second element
+  len /= 2;
+  qsort(pair, len, sizeof(keep), (void *)klatch);
+
+  // Pull out depends-on-self nodes, printing non-duplicate orpans
+  while (len) {
+    for (ii = 0, kk = ll = 2*len; ii<len; ii++) {
+      // First time through pull out depends-on-self nodes,
+      // else pull out nodes no other node depends on
+      if (first ? (long)strcmp(pair[2*ii], pair[2*ii+1])
+          : (long)bsearch(pair+2*ii-1, pair, len, sizeof(keep), (void *)klatch))
+        continue;
+
+      // Remove node from list, keeping at end for dupe killing
+      memcpy(keep, pair+2*ii, sizeof(keep));
+      memmove(pair+2*ii, pair+2*(ii+1), (--len-ii)*sizeof(keep));
+      ii--;
+
+      // depends-on-self nodes only count if nobody else depends on them
+      if (first && bsearch(keep, pair, len, sizeof(keep), (void *)klatch))
+        continue;
+
+      // Print non-duplicate nodes we removed
+      for (jj = kk; jj<ll; jj++) if (!strcmp(*keep, pair[jj])) break;
+      if (jj==ll) xprintf("1) %s\n", pair[--kk] = *keep);
+      if (bsearch(keep, pair, len, sizeof(keep), (void *)klatch)) continue;
+      for (jj = kk; jj<ll; jj++) if (!strcmp(keep[1], pair[jj])) break;
+      if (jj==ll) xprintf("2) %s\n", pair[--kk] = keep[1]);
+    }
+
+    // If we removed nothing, break;
+    if (first) first = 0;
+    else if (ll == 2*len) break;
+  }
+
+  // If we couldn't empty list, there's a cycle
+  if (len) error_msg("cycle from %s", *pair);
+}
+
+void tsort_main(void)
+{
+  loopfiles(toys.optargs, do_hersheba);
+}
-- 
2.39.2