Move query to its own subpackage (#152)

Move all the query system to its own package. The reason is to
avoid it to rely on unexported methods and structures, and move
it out of the main package since this is really not a core
feature. It is still tied to the toml.TomlTree and toml.Position
structures for now.

* Move query mechanism to its own subpackage
* Rename QueryResult to Result to avoid stutter
* Add query.CompileAndExecute

Fixes #116

query/query.go

@@ -0,0 +1,158 @@
+package query
+
+import (
+	"time"
+
+	"github.com/pelletier/go-toml"
+)
+
+// NodeFilterFn represents a user-defined filter function, for use with
+// Query.SetFilter().
+//
+// The return value of the function must indicate if 'node' is to be included
+// at this stage of the TOML path.  Returning true will include the node, and
+// returning false will exclude it.
+//
+// NOTE: Care should be taken to write script callbacks such that they are safe
+// to use from multiple goroutines.
+type NodeFilterFn func(node interface{}) bool
+
+// Result is the result of Executing a Query.
+type Result struct {
+	items     []interface{}
+	positions []toml.Position
+}
+
+// appends a value/position pair to the result set.
+func (r *Result) appendResult(node interface{}, pos toml.Position) {
+	r.items = append(r.items, node)
+	r.positions = append(r.positions, pos)
+}
+
+// Values is a set of values within a Result.  The order of values is not
+// guaranteed to be in document order, and may be different each time a query is
+// executed.
+func (r Result) Values() []interface{} {
+	return r.items
+}
+
+// Positions is a set of positions for values within a Result.  Each index
+// in Positions() corresponds to the entry in Value() of the same index.
+func (r Result) Positions() []toml.Position {
+	return r.positions
+}
+
+// runtime context for executing query paths
+type queryContext struct {
+	result       *Result
+	filters      *map[string]NodeFilterFn
+	lastPosition toml.Position
+}
+
+// generic path functor interface
+type pathFn interface {
+	setNext(next pathFn)
+	// it is the caller's responsibility to set the ctx.lastPosition before invoking call()
+	// node can be one of: *toml.TomlTree, []*toml.TomlTree, or a scalar
+	call(node interface{}, ctx *queryContext)
+}
+
+// A Query is the representation of a compiled TOML path.  A Query is safe
+// for concurrent use by multiple goroutines.
+type Query struct {
+	root    pathFn
+	tail    pathFn
+	filters *map[string]NodeFilterFn
+}
+
+func newQuery() *Query {
+	return &Query{
+		root:    nil,
+		tail:    nil,
+		filters: &defaultFilterFunctions,
+	}
+}
+
+func (q *Query) appendPath(next pathFn) {
+	if q.root == nil {
+		q.root = next
+	} else {
+		q.tail.setNext(next)
+	}
+	q.tail = next
+	next.setNext(newTerminatingFn()) // init the next functor
+}
+
+// Compile compiles a TOML path expression. The returned Query can be used
+// to match elements within a TomlTree and its descendants. See Execute.
+func Compile(path string) (*Query, error) {
+	return parseQuery(lexQuery(path))
+}
+
+// Execute executes a query against a TomlTree, and returns the result of the query.
+func (q *Query) Execute(tree *toml.TomlTree) *Result {
+	result := &Result{
+		items:     []interface{}{},
+		positions: []toml.Position{},
+	}
+	if q.root == nil {
+		result.appendResult(tree, tree.GetPosition(""))
+	} else {
+		ctx := &queryContext{
+			result:  result,
+			filters: q.filters,
+		}
+		ctx.lastPosition = tree.Position()
+		q.root.call(tree, ctx)
+	}
+	return result
+}
+
+// CompileAndExecute is a shorthand for Compile(path) followed by Execute(tree).
+func CompileAndExecute(path string, tree *toml.TomlTree) (*Result, error) {
+	query, err := Compile(path)
+	if err != nil {
+		return nil, err
+	}
+	return query.Execute(tree), nil
+}
+
+// SetFilter sets a user-defined filter function.  These may be used inside
+// "?(..)" query expressions to filter TOML document elements within a query.
+func (q *Query) SetFilter(name string, fn NodeFilterFn) {
+	if q.filters == &defaultFilterFunctions {
+		// clone the static table
+		q.filters = &map[string]NodeFilterFn{}
+		for k, v := range defaultFilterFunctions {
+			(*q.filters)[k] = v
+		}
+	}
+	(*q.filters)[name] = fn
+}
+
+var defaultFilterFunctions = map[string]NodeFilterFn{
+	"tree": func(node interface{}) bool {
+		_, ok := node.(*toml.TomlTree)
+		return ok
+	},
+	"int": func(node interface{}) bool {
+		_, ok := node.(int64)
+		return ok
+	},
+	"float": func(node interface{}) bool {
+		_, ok := node.(float64)
+		return ok
+	},
+	"string": func(node interface{}) bool {
+		_, ok := node.(string)
+		return ok
+	},
+	"time": func(node interface{}) bool {
+		_, ok := node.(time.Time)
+		return ok
+	},
+	"bool": func(node interface{}) bool {
+		_, ok := node.(bool)
+		return ok
+	},
+}