From 78400385c35ba9fc2d41cfbde1836e953e8a658c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Hugo=20H=C3=B6rnquist?= <hugo@lysator.liu.se>
Date: Mon, 20 Jul 2020 03:21:34 +0200
Subject: Add module for immutable directed graphs.

---
 module/util/graph.scm | 93 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 93 insertions(+)
 create mode 100644 module/util/graph.scm

diff --git a/module/util/graph.scm b/module/util/graph.scm
new file mode 100644
index 00000000..5bd86072
--- /dev/null
+++ b/module/util/graph.scm
@@ -0,0 +1,93 @@
+;;; Commentary:
+;; An immutable directed graph.
+;; Most operations are O(n), since there is no total
+;; order on symbols in scheme.
+;;; Code:
+
+(define-module (util graph)
+  :use-module (util)
+  :use-module (srfi srfi-1)
+  :use-module (srfi srfi-9 gnu))
+
+;; Immutable directed graph
+(define-immutable-record-type <graph>
+  (make-graph% nodes edges node-key-proc node-equal?)
+  graph?
+  (nodes graph-nodes)
+  (edges graph-edges) ; (list (symb . symb))
+  (node-key-proc node-key-proc) ; node → symb
+  (node-equal? node-equal?) ; node, node -> symb
+  )
+
+(define*-public (make-graph optional:
+                            (node-key-proc identity)
+                            (node-equal? eq?))
+  (make-graph% '() '() node-key-proc node-equal?))
+
+(define*-public (rebuild-graph optional: old-graph
+                               (nodes '()) (edges '()))
+  (make-graph% nodes edges
+               (if old-graph (node-key-proc old-graph) identity)
+               (if old-graph (node-equal? old-graph) eq?)))
+
+(define-public (graph-empty? graph)
+  (null? (graph-nodes graph)))
+
+;; Add node to graph. Adds directed edges from node to neighbours
+;; graph, node, (list node-key) → graph
+(define-public (add-node graph node edge-neighbours)
+  (rebuild-graph
+   graph
+   (lset-adjoin (node-equal? graph) (graph-nodes graph)
+                node)
+   (lset-union equal? (graph-edges graph)
+               (map (lambda (o) (cons ((node-key-proc graph) node) o))
+                    edge-neighbours))))
+
+;; get node by key
+(define-public (get-node graph key)
+  (find (lambda (node) (eq? key ((node-key-proc graph) node)))
+        (graph-nodes graph)))
+
+;; Remove node by @var{node-equal?}
+(define-public (remove-node graph node)
+  (rebuild-graph
+   graph
+   (remove (lambda (other) ((node-equal? graph) node other))
+           (graph-nodes graph))
+   (let ((key ((node-key-proc graph) node)))
+     (remove (lambda (edge) (or (eq? key (car edge))
+                           (eq? key (cdr edge))))
+             (graph-edges graph)))))
+
+;; NOTE this is O(n^2) (maybe, sort of?)
+;; Getting it faster would require building an index, which
+;; is hard since there isn't a total order on symbols.
+(define-public (find-node-without-dependencies graph)
+  (find (lambda (node)
+          (let ((key ((node-key-proc graph) node)))
+            (not (find (lambda (edge) (eq? key (car edge))) (graph-edges graph)))))
+        (graph-nodes graph)))
+
+;; graph → node x graph
+(define-public (find-and-remove-node-without-dependencies graph)
+  (let ((node (find-node-without-dependencies graph)))
+    (unless node
+      (throw 'graph-error 'find-and-remove-node-without-dependencies
+             "No node without dependencies in graph" '() graph))
+    (values node (remove-node graph node))))
+
+;; Assumes that the edges of the graph are dependencies.
+;; Returns a list of all nodes so that each node is after its dependants.
+;; A missing dependency (and probably a loop) is an error, and currently
+;; leads to some weird error messages.
+(define-public (resolve-dependency-graph graph)
+  (catch 'graph-error
+    (lambda ()
+      (let loop ((graph graph))
+        (if (graph-empty? graph)
+            '()
+            (let* ((node graph* (find-and-remove-node-without-dependencies graph)))
+              (cons node (loop graph*))))))
+    (lambda (err caller fmt args graph . data)
+      graph)))
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