added @mlubin's suggestions for indexing to preserve KeyIndex and oth…

…ers O(1)

src/common.jl

@@ -8,8 +8,6 @@ typealias AttributeDict Dict{UTF8String, Any}
 #
 ################################################
 
-vertex_index(v::Integer) = v
-
 immutable KeyVertex{K}
     index::Int
     key::K

src/graph.jl

@@ -56,7 +56,15 @@ num_edges(g::GenericGraph) = length(g.edges)
 edges(g::GenericGraph) = g.edges
 
 vertex_index(v::Integer, g::SimpleGraph) = (v <= g.vertices[end]? v: 0)
-vertex_index{V<:ProvidedVertexType}(v::V, g::GenericGraph{V}) = vertex_index(v,vertices(g))
+
+function vertex_index{V<:ProvidedVertexType}(v::V, g::GenericGraph{V})
+    if applicable(vertex_index, v)  # use O(1) if possible
+        return vertex_index(v)
+    else                            # use O(n)
+        return vertex_index(v,vertices(g))
+    end
+end
+
 edge_index{V,E}(e::E, g::GenericGraph{V,E}) = edge_index(e)
 
 out_edges{V}(v::V, g::GenericGraph{V}) = g.finclist[vertex_index(v, g)]
@@ -71,6 +79,7 @@ in_neighbors{V}(v::V, g::GenericGraph{V}) = SourceIterator(g, in_edges(v, g))
 # mutation
 
 function add_vertex!(g::SimpleGraph)
+    # ensure SimpleGraph indices are consecutive, allowing O(1) indexing
     v = g.vertices[end] + 1
     g.vertices = 1:v
     push!(g.finclist, Int[])