From 05d48bb7b4285c13c03bffe676934298dcbbc469 Mon Sep 17 00:00:00 2001
From: Alexandre Terrasa <alexandre@moz-code.org>
Date: Thu, 15 Oct 2015 17:31:56 -0400
Subject: [PATCH] lib/graphs: introduce PageRank computation on digraphs

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
---
 lib/graphs/pagerank.nit |   88 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 88 insertions(+)
 create mode 100644 lib/graphs/pagerank.nit

diff --git a/lib/graphs/pagerank.nit b/lib/graphs/pagerank.nit
new file mode 100644
index 0000000..5177e5a
--- /dev/null
+++ b/lib/graphs/pagerank.nit
@@ -0,0 +1,88 @@
+# This file is part of NIT (http://www.nitlanguage.org).
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Add PageRank computation for vertices in Digraph.
+module pagerank
+
+import graphs::digraph
+
+redef class Digraph[V]
+
+	# Compute PageRank for each vertex
+	#
+	# Details of the algorithm can be found in:
+	# > L. Page, S. Brin, R. Motwani, and T.Winograd.
+	# > **The pagerank citation ranking: Bringing order to the web.**
+	# > *Technical report, Stanford Digital Library Technologies Project, 1998*
+	#
+	# Example:
+	# ~~~
+	# var g = new HashDigraph[String]
+	# g.add_arc("A", "B")
+	# g.add_arc("A", "C")
+	# g.add_arc("B", "C")
+	# g.add_arc("C", "A")
+	# g.add_arc("D", "C")
+	#
+	# assert g.pagerank.join(", ", ":") == "A:1.488, B:0.782, C:1.575, D:0.15"
+	# ~~~
+	fun pagerank: PRMap[V] do
+		# `d` constant such as `initial_pagerank(node) == (1 - d) != 0`
+		var d = 0.85 # commonly-choosen value
+		# Init each node page rank with an initial_value
+		var values = new PRMap[V]
+		var vertices = self.vertices
+		for v in vertices do values[v] = 1.0 - d
+		# Compute page rank until convergence
+		var prev = new PRMap[V]
+		while not values.is_approx(prev, 0.001) do
+			prev = new PRMap[V].from(values)
+			for v in vertices do
+				var in_pr = 0.0
+				for o in predecessors(v) do
+					in_pr += values[o] / out_degree(o).to_f
+				end
+				values[v] = (1.0 - d) + d * in_pr
+			end
+		end
+		return values
+	end
+end
+
+# Map each Vertice of a Digraph to it's PageRank.
+#
+# See: `Digraph::pagerank`.
+class PRMap[V]
+	super HashMap[V, Float]
+
+	# Init `self` by copying `other` values.
+	init from(other: PRMap[V]) do
+		init
+		for k, v in other do self[k] = v
+	end
+
+	# Is `self` approximately equal to another PRMap?
+	#
+	# `self` is approximately equal to `o` if `o` contains all the key from `self`
+	# with the same values.
+	#
+	# Values equality is based on `Float::is_approx` with `precision`.
+	fun is_approx(o: SELF, precision: Float): Bool do
+		for k1, v1 in self do
+			if not o.has_key(k1) then return false
+			if not v1.is_approx(o[k1], precision) then return false
+		end
+		return true
+	end
+end
-- 
1.7.9.5