indexBy, which indexes a collection into a hashtable

lib/pure/collections/tables.nim

@@ -189,6 +189,16 @@ template dollarImpl(): stmt {.dirty.} =
 proc `$`*[A, B](t: TTable[A, B]): string =
   ## The `$` operator for hash tables.
   dollarImpl()
+  
+proc `==`*[A, B](s, t: TTable[A, B]): bool =
+  s.counter == t.counter and s.data == t.data
+  
+proc indexBy*[A, B, C](collection: A, index: proc(x: B): C): TTable[C, B] =
+  ## Index the collection with the proc provided.
+  # TODO: As soon as supported, change collection: A to collection: A[B]
+  result = initTable[C, B]()
+  for item in collection:
+    result[index(item)] = item
 
 # ------------------------------ ordered table ------------------------------
 

tests/collections/ttables.nim

@@ -0,0 +1,22 @@
+import tables
+
+doAssert indexBy(newSeq[int](), proc(x: int):int = x) == initTable[int, int](), "empty int table"
+
+var tbl1 = initTable[int, int]()
+tbl1.add(1,1)
+tbl1.add(2,2)
+doAssert indexBy(@[1,2], proc(x: int):int = x) == tbl1, "int table"
+
+type
+  TElem = object
+    foo: int
+    bar: string
+    
+let
+  elem1 = TElem(foo: 1, bar: "bar")
+  elem2 = TElem(foo: 2, bar: "baz")
+  
+var tbl2 = initTable[string, TElem]()
+tbl2.add("bar", elem1)
+tbl2.add("baz", elem2)
+doAssert indexBy(@[elem1,elem2], proc(x: TElem): string = x.bar) == tbl2, "element table"