Fix index and slice expressions for query (#405)

* Fix index and slice expressions for query

Support negative step for slice expressions

query/README.md

@@ -0,0 +1,201 @@
+# Query package
+
+## Overview
+
+Package query performs JSONPath-like queries on a TOML document.
+
+The query path implementation is based loosely on the JSONPath specification:
+http://goessner.net/articles/JsonPath/.
+
+The idea behind a query path is to allow quick access to any element, or set
+of elements within TOML document, with a single expression.
+
+```go
+result, err := query.CompileAndExecute("$.foo.bar.baz", tree)
+```
+
+This is roughly equivalent to:
+
+```go
+next := tree.Get("foo")
+if next != nil {
+    next = next.Get("bar")
+    if next != nil {
+        next = next.Get("baz")
+    }
+}
+result := next
+```
+
+err is nil if any parsing exception occurs.
+
+If no node in the tree matches the query, result will simply contain an empty list of
+items.
+
+As illustrated above, the query path is much more efficient, especially since
+the structure of the TOML file can vary. Rather than making assumptions about
+a document's structure, a query allows the programmer to make structured
+requests into the document, and get zero or more values as a result.
+
+## Query syntax
+
+The syntax of a query begins with a root token, followed by any number
+sub-expressions:
+
+```
+$
+                    Root of the TOML tree. This must always come first.
+.name
+                    Selects child of this node, where 'name' is a TOML key
+                    name.
+['name']
+                    Selects child of this node, where 'name' is a string
+                    containing a TOML key name.
+[index]
+                    Selcts child array element at 'index'.
+..expr
+                    Recursively selects all children, filtered by an a union,
+                    index, or slice expression.
+..*
+                    Recursive selection of all nodes at this point in the
+                    tree.
+.*
+                    Selects all children of the current node.
+[expr,expr]
+                    Union operator - a logical 'or' grouping of two or more
+                    sub-expressions: index, key name, or filter.
+[start:end:step]
+                    Slice operator - selects array elements from start to
+                    end-1, at the given step. All three arguments are
+                    optional.
+[?(filter)]
+                    Named filter expression - the function 'filter' is
+                    used to filter children at this node.
+```
+
+## Query Indexes And Slices
+
+Index expressions perform no bounds checking, and will contribute no
+values to the result set if the provided index or index range is invalid.
+Negative indexes represent values from the end of the array, counting backwards.
+
+```go
+// select the last index of the array named 'foo'
+query.CompileAndExecute("$.foo[-1]", tree)
+```
+
+Slice expressions are supported, by using ':' to separate a start/end index pair.
+
+```go
+// select up to the first five elements in the array
+query.CompileAndExecute("$.foo[0:5]", tree)
+```
+
+Slice expressions also allow negative indexes for the start and stop
+arguments.
+
+```go
+// select all array elements except the last one.
+query.CompileAndExecute("$.foo[0:-1]", tree)
+```
+
+Slice expressions may have an optional stride/step parameter:
+
+```go
+// select every other element
+query.CompileAndExecute("$.foo[0::2]", tree)
+```
+
+Slice start and end parameters are also optional:
+
+```go
+// these are all equivalent and select all the values in the array
+query.CompileAndExecute("$.foo[:]", tree)
+query.CompileAndExecute("$.foo[::]", tree)
+query.CompileAndExecute("$.foo[::1]", tree)
+query.CompileAndExecute("$.foo[0:]", tree)
+query.CompileAndExecute("$.foo[0::]", tree)
+query.CompileAndExecute("$.foo[0::1]", tree)
+```
+
+## Query Filters
+
+Query filters are used within a Union [,] or single Filter [] expression.
+A filter only allows nodes that qualify through to the next expression,
+and/or into the result set.
+
+```go
+// returns children of foo that are permitted by the 'bar' filter.
+query.CompileAndExecute("$.foo[?(bar)]", tree)
+```
+
+There are several filters provided with the library:
+
+```
+tree
+        Allows nodes of type Tree.
+int
+        Allows nodes of type int64.
+float
+        Allows nodes of type float64.
+string
+        Allows nodes of type string.
+time
+        Allows nodes of type time.Time.
+bool
+        Allows nodes of type bool.
+```
+
+## Query Results
+
+An executed query returns a Result object. This contains the nodes
+in the TOML tree that qualify the query expression. Position information
+is also available for each value in the set.
+
+```go
+// display the results of a query
+results := query.CompileAndExecute("$.foo.bar.baz", tree)
+for idx, value := results.Values() {
+    fmt.Println("%v: %v", results.Positions()[idx], value)
+}
+```
+
+## Compiled Queries
+
+Queries may be executed directly on a Tree object, or compiled ahead
+of time and executed discretely. The former is more convenient, but has the
+penalty of having to recompile the query expression each time.
+
+```go
+// basic query
+results := query.CompileAndExecute("$.foo.bar.baz", tree)
+
+// compiled query
+query, err := toml.Compile("$.foo.bar.baz")
+results := query.Execute(tree)
+
+// run the compiled query again on a different tree
+moreResults := query.Execute(anotherTree)
+```
+
+## User Defined Query Filters
+
+Filter expressions may also be user defined by using the SetFilter()
+function on the Query object. The function must return true/false, which
+signifies if the passed node is kept or discarded, respectively.
+
+```go
+// create a query that references a user-defined filter
+query, _ := query.Compile("$[?(bazOnly)]")
+
+// define the filter, and assign it to the query
+query.SetFilter("bazOnly", func(node interface{}) bool{
+    if tree, ok := node.(*Tree); ok {
+        return tree.Has("baz")
+    }
+    return false // reject all other node types
+})
+
+// run the query
+query.Execute(tree)
+```