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package main
import (
type cursor_context struct {
decl *decl
partial string
struct_field bool
decl_import bool
// store expression that was supposed to be deduced to "decl", however
// if decl is nil, then deduction failed, we could try to resolve it to
// unimported package instead
expr ast.Expr
type token_iterator struct {
tokens []token_item
token_index int
type token_item struct {
off int
tok token.Token
lit string
func (i token_item) literal() string {
if i.tok.IsLiteral() {
return i.lit
} else {
return i.tok.String()
return ""
func new_token_iterator(src []byte, cursor int) token_iterator {
tokens := make([]token_item, 0, 1000)
var s scanner.Scanner
fset := token.NewFileSet()
file := fset.AddFile("", fset.Base(), len(src))
s.Init(file, src, nil, 0)
for {
pos, tok, lit := s.Scan()
off := fset.Position(pos).Offset
if tok == token.EOF || cursor <= off {
tokens = append(tokens, token_item{
off: off,
tok: tok,
lit: lit,
return token_iterator{
tokens: tokens,
token_index: len(tokens) - 1,
func (this *token_iterator) token() token_item {
return this.tokens[this.token_index]
func (this *token_iterator) go_back() bool {
if this.token_index <= 0 {
return false
return true
var bracket_pairs_map = map[token.Token]token.Token{
token.RPAREN: token.LPAREN,
token.RBRACK: token.LBRACK,
token.RBRACE: token.LBRACE,
func (ti *token_iterator) skip_to_left(left, right token.Token) bool {
if ti.token().tok == left {
return true
balance := 1
for balance != 0 {
if !ti.go_back() {
return false
switch ti.token().tok {
case right:
case left:
return true
// when the cursor is at the ')' or ']' or '}', move the cursor to an opposite
// bracket pair, this functions takes nested bracket pairs into account
func (this *token_iterator) skip_to_balanced_pair() bool {
right := this.token().tok
left := bracket_pairs_map[right]
return this.skip_to_left(left, right)
// Move the cursor to the open brace of the current block, taking nested blocks
// into account.
func (this *token_iterator) skip_to_left_curly() bool {
return this.skip_to_left(token.LBRACE, token.RBRACE)
// Extract the type expression right before the enclosing curly bracket block.
// Examples (# - the cursor):
// &lib.Struct{Whatever: 1, Hel#} // returns "lib.Struct"
// X{#} // returns X
// The idea is that we check if this type expression is a type and it is, we
// can apply special filtering for autocompletion results.
// Sadly, this doesn't cover anonymous structs.
func (ti *token_iterator) extract_struct_type() string {
if !ti.skip_to_left_curly() {
return ""
if !ti.go_back() {
return ""
if ti.token().tok != token.IDENT {
return ""
b := ti.token().literal()
if !ti.go_back() {
return b
if ti.token().tok != token.PERIOD {
return b
if !ti.go_back() {
return b
if ti.token().tok != token.IDENT {
return b
return ti.token().literal() + "." + b
// Starting from the token under the cursor move back and extract something
// that resembles a valid Go primary expression. Examples of primary expressions
// from Go spec:
// x
// 2
// (s + ".txt")
// f(3.1415, true)
// Point{1, 2}
// m["foo"]
// s[i : j + 1]
// obj.color
// f.p[i].x()
// As you can see we can move through all of them using balanced bracket
// matching and applying simple rules
// E.g.
// Point{1, 2}.m["foo"].s[i : j + 1].MethodCall(a, func(a, b int) int { return a + b }).
// Can be seen as:
// Point{ }.m[ ].s[ ].MethodCall( ).
// Which boils the rules down to these connected via dots:
// ident
// ident[]
// ident{}
// ident()
// Of course there are also slightly more complicated rules for brackets:
// ident{}.ident()[5][4](), etc.
func (this *token_iterator) extract_go_expr() string {
orig := this.token_index
// Contains the type of the previously scanned token (initialized with
// the token right under the cursor). This is the token to the *right* of
// the current one.
prev := this.token().tok
for {
if !this.go_back() {
return token_items_to_string(this.tokens[:orig])
switch this.token().tok {
case token.PERIOD:
// If the '.' is not followed by IDENT, it's invalid.
if prev != token.IDENT {
break loop
case token.IDENT:
// Valid tokens after IDENT are '.', '[', '{' and '('.
switch prev {
case token.PERIOD, token.LBRACK, token.LBRACE, token.LPAREN:
// all ok
break loop
case token.RBRACE:
// This one can only be a part of type initialization, like:
// Dummy{}.Hello()
// It is valid Go if Hello method is defined on a non-pointer receiver.
if prev != token.PERIOD {
break loop
case token.RPAREN, token.RBRACK:
// After ']' and ')' their opening counterparts are valid '[', '(',
// as well as the dot.
switch prev {
case token.PERIOD, token.LBRACK, token.LPAREN:
// all ok
break loop
break loop
prev = this.token().tok
expr := token_items_to_string(this.tokens[this.token_index+1 : orig])
if *g_debug {
log.Printf("extracted expression tokens: %s", expr)
return expr
// Given a slice of token_item, reassembles them into the original literal
// expression.
func token_items_to_string(tokens []token_item) string {
var buf bytes.Buffer
for _, t := range tokens {
return buf.String()
// this function is called when the cursor is at the '.' and you need to get the
// declaration before that dot
func (c *auto_complete_context) deduce_cursor_decl(iter *token_iterator) (*decl, ast.Expr) {
expr, err := parser.ParseExpr(iter.extract_go_expr())
if err != nil {
return nil, nil
return expr_to_decl(expr, c.current.scope), expr
// try to find and extract the surrounding struct literal type
func (c *auto_complete_context) deduce_struct_type_decl(iter *token_iterator) *decl {
typ := iter.extract_struct_type()
if typ == "" {
return nil
expr, err := parser.ParseExpr(typ)
if err != nil {
return nil
decl := type_to_decl(expr, c.current.scope)
if decl == nil {
return nil
if _, ok := decl.typ.(*ast.StructType); !ok {
return nil
return decl
// Entry point from autocompletion, the function looks at text before the cursor
// and figures out the declaration the cursor is on. This declaration is
// used in filtering the resulting set of autocompletion suggestions.
func (c *auto_complete_context) deduce_cursor_context(file []byte, cursor int) (cursor_context, bool) {
if cursor <= 0 {
return cursor_context{}, true
iter := new_token_iterator(file, cursor)
if len(iter.tokens) == 0 {
return cursor_context{}, false
// figure out what is just before the cursor
switch tok := iter.token(); tok.tok {
case token.STRING:
// make sure cursor is inside the string
s := tok.literal()
if len(s) > 1 && s[len(s)-1] == '"' && <= cursor {
return cursor_context{}, true
// now figure out if inside an import declaration
var ptok = token.STRING
for iter.go_back() {
itok := iter.token().tok
switch itok {
case token.STRING:
switch ptok {
case token.SEMICOLON, token.IDENT, token.PERIOD:
return cursor_context{}, true
case token.LPAREN, token.SEMICOLON:
switch ptok {
case token.STRING, token.IDENT, token.PERIOD:
return cursor_context{}, true
case token.IDENT, token.PERIOD:
switch ptok {
case token.STRING:
return cursor_context{}, true
case token.IMPORT:
switch ptok {
case token.STRING, token.IDENT, token.PERIOD, token.LPAREN:
path_len := cursor -
path := s[1:path_len]
return cursor_context{decl_import: true, partial: path}, true
return cursor_context{}, true
return cursor_context{}, true
ptok = itok
case token.PERIOD:
// we're '<whatever>.'
// figure out decl, Partial is ""
decl, expr := c.deduce_cursor_decl(&iter)
return cursor_context{decl: decl, expr: expr}, decl != nil
case token.IDENT, token.TYPE, token.CONST, token.VAR, token.FUNC, token.PACKAGE:
// we're '<whatever>.<ident>'
// parse <ident> as Partial and figure out decl
var partial string
if tok.tok == token.IDENT {
// Calculate the offset of the cursor position within the identifier.
// For instance, if we are 'ab#c', we want partial_len = 2 and partial = ab.
partial_len := cursor -
// If it happens that the cursor is past the end of the literal,
// means there is a space between the literal and the cursor, think
// of it as no context, because that's what it really is.
if partial_len > len(tok.literal()) {
return cursor_context{}, true
partial = tok.literal()[0:partial_len]
} else {
// Do not try to truncate if it is not an identifier.
partial = tok.literal()
switch iter.token().tok {
case token.PERIOD:
decl, expr := c.deduce_cursor_decl(&iter)
return cursor_context{decl: decl, partial: partial, expr: expr}, decl != nil
case token.COMMA, token.LBRACE:
// This can happen for struct fields:
// &Struct{Hello: 1, Wor#} // (# - the cursor)
// Let's try to find the struct type
decl := c.deduce_struct_type_decl(&iter)
return cursor_context{
decl: decl,
partial: partial,
struct_field: decl != nil,
}, true
return cursor_context{partial: partial}, true
case token.COMMA, token.LBRACE:
// Try to parse the current expression as a structure initialization.
decl := c.deduce_struct_type_decl(&iter)
return cursor_context{
decl: decl,
partial: "",
struct_field: decl != nil,
}, true
return cursor_context{}, true
// Decl deduction failed, but we're on "<ident>.", this ident can be an
// unexported package, let's try to match the ident against a set of known
// packages and if it matches try to import it.
// TODO: Right now I've made a static list of built-in packages, but in theory
// we could scan all GOPATH packages as well. Now, don't forget that default
// package name has nothing to do with package file name, that's why we need to
// scan the packages. And many of them will have conflicts. Can we make a smart
// prediction algorithm which will prefer certain packages over another ones?
func resolveKnownPackageIdent(ident string, filename string, context *package_lookup_context) *decl {
importPath, ok := knownPackageIdents[ident]
if !ok {
return nil
path, ok := abs_path_for_package(filename, importPath, context)
if !ok {
return nil
p := new_package_file_cache(path)
return p.main
var knownPackageIdents = map[string]string{
"adler32": "hash/adler32",
"aes": "crypto/aes",
"ascii85": "encoding/ascii85",
"asn1": "encoding/asn1",
"ast": "go/ast",
"atomic": "sync/atomic",
"base32": "encoding/base32",
"base64": "encoding/base64",
"big": "math/big",
"binary": "encoding/binary",
"bufio": "bufio",
"build": "go/build",
"bytes": "bytes",
"bzip2": "compress/bzip2",
"cgi": "net/http/cgi",
"cgo": "runtime/cgo",
"cipher": "crypto/cipher",
"cmplx": "math/cmplx",
"color": "image/color",
"constant": "go/constant",
"context": "context",
"cookiejar": "net/http/cookiejar",
"crc32": "hash/crc32",
"crc64": "hash/crc64",
"crypto": "crypto",
"csv": "encoding/csv",
"debug": "runtime/debug",
"des": "crypto/des",
"doc": "go/doc",
"draw": "image/draw",
"driver": "database/sql/driver",
"dsa": "crypto/dsa",
"dwarf": "debug/dwarf",
"ecdsa": "crypto/ecdsa",
"elf": "debug/elf",
"elliptic": "crypto/elliptic",
"encoding": "encoding",
"errors": "errors",
"exec": "os/exec",
"expvar": "expvar",
"fcgi": "net/http/fcgi",
"filepath": "path/filepath",
"flag": "flag",
"flate": "compress/flate",
"fmt": "fmt",
"fnv": "hash/fnv",
"format": "go/format",
"gif": "image/gif",
"gob": "encoding/gob",
"gosym": "debug/gosym",
"gzip": "compress/gzip",
"hash": "hash",
"heap": "container/heap",
"hex": "encoding/hex",
"hmac": "crypto/hmac",
"hpack": "vendor/golang_org/x/net/http2/hpack",
"html": "html",
"http": "net/http",
"httplex": "vendor/golang_org/x/net/lex/httplex",
"httptest": "net/http/httptest",
"httptrace": "net/http/httptrace",
"httputil": "net/http/httputil",
"image": "image",
"importer": "go/importer",
"io": "io",
"iotest": "testing/iotest",
"ioutil": "io/ioutil",
"jpeg": "image/jpeg",
"json": "encoding/json",
"jsonrpc": "net/rpc/jsonrpc",
"list": "container/list",
"log": "log",
"lzw": "compress/lzw",
"macho": "debug/macho",
"mail": "net/mail",
"math": "math",
"md5": "crypto/md5",
"mime": "mime",
"multipart": "mime/multipart",
"net": "net",
"os": "os",
"palette": "image/color/palette",
"parse": "text/template/parse",
"parser": "go/parser",
"path": "path",
"pe": "debug/pe",
"pem": "encoding/pem",
"pkix": "crypto/x509/pkix",
"plan9obj": "debug/plan9obj",
"png": "image/png",
"pprof": "net/http/pprof",
"printer": "go/printer",
"quick": "testing/quick",
"quotedprintable": "mime/quotedprintable",
"race": "runtime/race",
"rand": "math/rand",
"rc4": "crypto/rc4",
"reflect": "reflect",
"regexp": "regexp",
"ring": "container/ring",
"rpc": "net/rpc",
"rsa": "crypto/rsa",
"runtime": "runtime",
"scanner": "text/scanner",
"sha1": "crypto/sha1",
"sha256": "crypto/sha256",
"sha512": "crypto/sha512",
"signal": "os/signal",
"smtp": "net/smtp",
"sort": "sort",
"sql": "database/sql",
"strconv": "strconv",
"strings": "strings",
"subtle": "crypto/subtle",
"suffixarray": "index/suffixarray",
"sync": "sync",
"syntax": "regexp/syntax",
"syscall": "syscall",
"syslog": "log/syslog",
"tabwriter": "text/tabwriter",
"tar": "archive/tar",
"template": "html/template",
"testing": "testing",
"textproto": "net/textproto",
"time": "time",
"tls": "crypto/tls",
"token": "go/token",
"trace": "runtime/trace",
"types": "go/types",
"unicode": "unicode",
"url": "net/url",
"user": "os/user",
"utf16": "unicode/utf16",
"utf8": "unicode/utf8",
"x509": "crypto/x509",
"xml": "encoding/xml",
"zip": "archive/zip",
"zlib": "compress/zlib",
//"scanner": "go/scanner", // DUP: prefer text/scanner
//"template": "text/template", // DUP: prefer html/template
//"pprof": "runtime/pprof", // DUP: prefer net/http/pprof
//"rand": "crypto/rand", // DUP: prefer math/rand