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vendor/github.com/tidwall/btree/README.md

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+BTree implementation for Go
+===========================
+
+![Travis CI Build Status](https://api.travis-ci.org/tidwall/btree.svg?branch=master)
+[![GoDoc](https://godoc.org/github.com/tidwall/btree?status.svg)](https://godoc.org/github.com/tidwall/btree)
+
+This package provides an in-memory B-Tree implementation for Go, useful as
+an ordered, mutable data structure.
+
+This is a fork of the wonderful [google/btree](https://github.com/google/btree) package. It's has all the same great features and adds a few more.
+
+- Descend* functions for iterating backwards.
+- Iteration performance boost.
+- User defined context.
+
+User defined context
+--------------------
+This is a great new feature that allows for entering the same item into multiple B-trees, and each B-tree have a different ordering formula.
+
+For example:
+
+```go
+package main
+
+import (
+	"fmt"
+
+	"github.com/tidwall/btree"
+)
+
+type Item struct {
+	Key, Val string
+}
+
+func (i1 *Item) Less(item btree.Item, ctx interface{}) bool {
+	i2 := item.(*Item)
+	switch tag := ctx.(type) {
+	case string:
+		if tag == "vals" {
+			if i1.Val < i2.Val {
+				return true
+			} else if i1.Val > i2.Val {
+				return false
+			}
+			// Both vals are equal so we should fall though
+			// and let the key comparison take over.
+		}
+	}
+	return i1.Key < i2.Key
+}
+
+func main() {
+
+	// Create a tree for keys and a tree for values.
+	// The "keys" tree will be sorted on the Keys field.
+	// The "values" tree will be sorted on the Values field.
+	keys := btree.New(16, "keys")
+	vals := btree.New(16, "vals")
+
+	// Create some items.
+	users := []*Item{
+		&Item{Key: "user:1", Val: "Jane"},
+		&Item{Key: "user:2", Val: "Andy"},
+		&Item{Key: "user:3", Val: "Steve"},
+		&Item{Key: "user:4", Val: "Andrea"},
+		&Item{Key: "user:5", Val: "Janet"},
+		&Item{Key: "user:6", Val: "Andy"},
+	}
+
+	// Insert each user into both trees
+	for _, user := range users {
+		keys.ReplaceOrInsert(user)
+		vals.ReplaceOrInsert(user)
+	}
+
+	// Iterate over each user in the key tree
+	keys.Ascend(func(item btree.Item) bool {
+		kvi := item.(*Item)
+		fmt.Printf("%s %s\n", kvi.Key, kvi.Val)
+		return true
+	})
+
+	fmt.Printf("\n")
+	// Iterate over each user in the val tree
+	vals.Ascend(func(item btree.Item) bool {
+		kvi := item.(*Item)
+		fmt.Printf("%s %s\n", kvi.Key, kvi.Val)
+		return true
+	})
+}
+
+// Should see the results
+/*
+user:1 Jane
+user:2 Andy
+user:3 Steve
+user:4 Andrea
+user:5 Janet
+user:6 Andy
+
+user:4 Andrea
+user:2 Andy
+user:6 Andy
+user:1 Jane
+user:3 Steve
+*/
+```