vars.go

package js

import (
	"bytes"
	"sort"

	"github.com/tdewolff/parse/v2/js"
)

const identStartLen = 54
const identContinueLen = 64

type renamer struct {
	identStart    []byte
	identContinue []byte
	identOrder    map[byte]int
	reserved      map[string]struct{}
	rename        bool
}

func newRenamer(rename, useCharFreq bool) *renamer {
	reserved := make(map[string]struct{}, len(js.Keywords))
	for name := range js.Keywords {
		reserved[name] = struct{}{}
	}
	identStart := []byte("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_$")
	identContinue := []byte("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_$0123456789")
	if useCharFreq {
		// sorted based on character frequency of a collection of JS samples
		identStart = []byte("etnsoiarclduhmfpgvbjy_wOxCEkASMFTzDNLRPHIBV$WUKqYGXQZJ")
		identContinue = []byte("etnsoiarcldu14023hm8f6pg57v9bjy_wOxCEkASMFTzDNLRPHIBV$WUKqYGXQZJ")
	}
	if len(identStart) != identStartLen || len(identContinue) != identContinueLen {
		panic("bad identStart or identContinue lengths")
	}
	identOrder := map[byte]int{}
	for i, c := range identStart {
		identOrder[c] = i
	}
	return &renamer{
		identStart:    identStart,
		identContinue: identContinue,
		identOrder:    identOrder,
		reserved:      reserved,
		rename:        rename,
	}
}

func (r *renamer) renameScope(scope js.Scope) {
	if !r.rename {
		return
	}

	i := 0
	// keep function argument declaration order to improve GZIP compression
	sort.Sort(js.VarsByUses(scope.Declared[scope.NumFuncArgs:]))
	for _, v := range scope.Declared {
		v.Data = r.getName(v.Data, i)
		i++
		for r.isReserved(v.Data, scope.Undeclared) {
			v.Data = r.getName(v.Data, i)
			i++
		}
	}
}

func (r *renamer) isReserved(name []byte, undeclared js.VarArray) bool {
	if 1 < len(name) { // there are no keywords or known globals that are one character long
		if _, ok := r.reserved[string(name)]; ok {
			return true
		}
	}
	for _, v := range undeclared {
		for v.Link != nil {
			v = v.Link
		}
		if bytes.Equal(v.Data, name) {
			return true
		}
	}
	return false
}

func (r *renamer) getIndex(name []byte) int {
	index := 0
NameLoop:
	for i := len(name) - 1; 0 <= i; i-- {
		chars := r.identContinue
		if i == 0 {
			chars = r.identStart
			index *= identStartLen
		} else {
			index *= identContinueLen
		}
		for j, c := range chars {
			if name[i] == c {
				index += j
				continue NameLoop
			}
		}
		return -1
	}
	for n := 0; n < len(name)-1; n++ {
		offset := identStartLen
		for i := 0; i < n; i++ {
			offset *= identContinueLen
		}
		index += offset
	}
	return index
}

func (r *renamer) getName(name []byte, index int) []byte {
	// Generate new names for variables where the last character is (a-zA-Z$_) and others are (a-zA-Z).
	// Thus we can have 54 one-character names and 52*54=2808 two-character names for every branch leaf.
	// That is sufficient for virtually all input.

	// one character
	if index < identStartLen {
		name[0] = r.identStart[index]
		return name[:1]
	}
	index -= identStartLen

	// two characters or more
	n := 2
	for {
		offset := identStartLen
		for i := 0; i < n-1; i++ {
			offset *= identContinueLen
		}
		if index < offset {
			break
		}
		index -= offset
		n++
	}

	if cap(name) < n {
		name = make([]byte, n)
	} else {
		name = name[:n]
	}
	name[0] = r.identStart[index%identStartLen]
	index /= identStartLen
	for i := 1; i < n; i++ {
		name[i] = r.identContinue[index%identContinueLen]
		index /= identContinueLen
	}
	return name
}

////////////////////////////////////////////////////////////////

func hasDefines(v *js.VarDecl) bool {
	for _, item := range v.List {
		if item.Default != nil {
			return true
		}
	}
	return false
}

func bindingVars(ibinding js.IBinding) (vs []*js.Var) {
	switch binding := ibinding.(type) {
	case *js.Var:
		vs = append(vs, binding)
	case *js.BindingArray:
		for _, item := range binding.List {
			if item.Binding != nil {
				vs = append(vs, bindingVars(item.Binding)...)
			}
		}
		if binding.Rest != nil {
			vs = append(vs, bindingVars(binding.Rest)...)
		}
	case *js.BindingObject:
		for _, item := range binding.List {
			if item.Value.Binding != nil {
				vs = append(vs, bindingVars(item.Value.Binding)...)
			}
		}
		if binding.Rest != nil {
			vs = append(vs, binding.Rest)
		}
	}
	return
}

func addDefinition(decl *js.VarDecl, binding js.IBinding, value js.IExpr, forward bool) {
	if decl.TokenType != js.ErrorToken {
		// see if not already defined in variable declaration list
		// if forward is set, binding=value comes before decl, otherwise the reverse holds true
		vars := bindingVars(binding)

		// remove variables in destination
	RemoveVarsLoop:
		for _, vbind := range vars {
			for i, item := range decl.List {
				if v, ok := item.Binding.(*js.Var); ok && item.Default == nil && v == vbind {
					v.Uses--
					decl.List = append(decl.List[:i], decl.List[i+1:]...)
					continue RemoveVarsLoop
				}
			}

			if value != nil {
				// variable declaration must be somewhere else, find and remove it
				for _, decl2 := range decl.Scope.Func.VarDecls {
					if !decl2.InForInOf {
						for i, item := range decl2.List {
							if v, ok := item.Binding.(*js.Var); ok && item.Default == nil && v == vbind {
								v.Uses--
								decl2.List = append(decl2.List[:i], decl2.List[i+1:]...)
								continue RemoveVarsLoop
							}
						}
					}
				}
			}
		}
	}

	// add declaration to destination
	item := js.BindingElement{Binding: binding, Default: value}
	if forward {
		decl.List = append([]js.BindingElement{item}, decl.List...)
	} else {
		decl.List = append(decl.List, item)
	}
}

func mergeVarDecls(dst, src *js.VarDecl, forward bool) {
	// Merge var declarations by moving declarations from src to dst. If forward is set, src comes first and dst after, otherwise the order is reverse.
	if forward {
		// reverse order so we can iterate from beginning to end, sometimes addDefinition may remove another declaration in the src list
		n := len(src.List) - 1
		for j := 0; j < len(src.List)/2; j++ {
			src.List[j], src.List[n-j] = src.List[n-j], src.List[j]
		}
	}
	for j := 0; j < len(src.List); j++ {
		addDefinition(dst, src.List[j].Binding, src.List[j].Default, forward)
	}
	src.List = src.List[:0]
}

func mergeVarDeclExprStmt(decl *js.VarDecl, exprStmt *js.ExprStmt, forward bool) bool {
	// Merge var declarations with an assignment expression. If forward is set than expr comes first and decl after, otherwise the order is reverse.
	if decl2, ok := exprStmt.Value.(*js.VarDecl); ok {
		// this happens when a variable declarations is converted to an expression due to hoisting
		mergeVarDecls(decl, decl2, forward)
		return true
	} else if commaExpr, ok := exprStmt.Value.(*js.CommaExpr); ok {
		n := 0
		for i := 0; i < len(commaExpr.List); i++ {
			item := commaExpr.List[i]
			if forward {
				item = commaExpr.List[len(commaExpr.List)-i-1]
			}
			if src, ok := item.(*js.VarDecl); ok {
				// this happens when a variable declarations is converted to an expression due to hoisting
				mergeVarDecls(decl, src, forward)
				n++
				continue
			} else if binaryExpr, ok := item.(*js.BinaryExpr); ok && binaryExpr.Op == js.EqToken {
				if v, ok := binaryExpr.X.(*js.Var); ok && v.Decl == js.VariableDecl {
					addDefinition(decl, v, binaryExpr.Y, forward)
					n++
					continue
				}
			}
			break
		}
		merge := n == len(commaExpr.List)
		if !forward {
			commaExpr.List = commaExpr.List[n:]
		} else {
			commaExpr.List = commaExpr.List[:len(commaExpr.List)-n]
		}
		return merge
	} else if binaryExpr, ok := exprStmt.Value.(*js.BinaryExpr); ok && binaryExpr.Op == js.EqToken {
		if v, ok := binaryExpr.X.(*js.Var); ok && v.Decl == js.VariableDecl {
			addDefinition(decl, v, binaryExpr.Y, forward)
			return true
		}
	}
	return false
}

func (m *jsMinifier) countHoistLength(ibinding js.IBinding) int {
	if !m.o.KeepVarNames {
		return len(bindingVars(ibinding)) * 2 // assume that var name will be of length one, +1 for the comma
	}

	n := 0
	for _, v := range bindingVars(ibinding) {
		n += len(v.Data) + 1 // +1 for the comma when added to other declaration
	}
	return n
}

func (m *jsMinifier) hoistVars(body *js.BlockStmt) {
	// Hoist all variable declarations in the current module/function scope to the variable
	// declaration that reduces file size the most. All other declarations are converted to
	// expressions and their variable names are copied to the only remaining declaration.
	// This is possible because an ArrayBindingPattern and ObjectBindingPattern can be converted to
	// an ArrayLiteral or ObjectLiteral respectively, as they are supersets of the BindingPatterns.
	if 1 < len(body.Scope.VarDecls) {
		// Select which variable declarations will be hoisted (convert to expression) and which not
		best := 0
		scores := make([]int, len(body.Scope.VarDecls)) // savings if hoisting target
		hoist := make([]bool, len(body.Scope.VarDecls))
		for i, varDecl := range body.Scope.VarDecls {
			hoist[i] = true
			if varDecl.InForInOf {
				continue
			}

			// variable names in for-in or for-of cannot be removed
			n := 0        // total number of vars with decls
			score := 3    // "var"
			nArrays := 0  // of which lhs arrays
			nObjects := 0 // of which lhs objects
			hasDefinitions := false
			for j, item := range varDecl.List {
				if item.Default != nil {
					// move arrays/objects to the front (saves a space)
					if _, ok := item.Binding.(*js.BindingObject); ok {
						if j != 0 && nArrays == 0 && nObjects == 0 {
							varDecl.List[0], varDecl.List[j] = varDecl.List[j], varDecl.List[0]
						}
						nObjects++
					} else if _, ok := item.Binding.(*js.BindingArray); ok {
						if j != 0 && nArrays == 0 && nObjects == 0 {
							varDecl.List[0], varDecl.List[j] = varDecl.List[j], varDecl.List[0]
						}
						nArrays++
					}
					score -= m.countHoistLength(item.Binding) // var names and commas
					hasDefinitions = true
					n++
				}
			}
			if nArrays == 0 && nObjects == 0 {
				score++ // required space after var
			}
			if !hasDefinitions && varDecl.InFor {
				score-- // semicolon can be reused
			}
			if nObjects != 0 && !varDecl.InFor && nObjects == n {
				// required parenthesis around braces to not confound it with a block statement
				score -= 2
			}
			if score < scores[best] || body.Scope.VarDecls[best].InForInOf {
				// select var decl that reduces the least when hoist target
				best = i
			}
			if score < 0 {
				// don't hoist if it increases the amount of characters
				hoist[i] = false
			}
			scores[i] = score
		}
		if body.Scope.VarDecls[best].InForInOf {
			// no savings possible
			return
		}

		decl := body.Scope.VarDecls[best]
		if 10000 < len(decl.List) {
			return
		}
		hoist[best] = false

		// get original declarations
		orig := []*js.Var{}
		for _, item := range decl.List {
			orig = append(orig, bindingVars(item.Binding)...)
		}

		// hoist other variable declarations in this function scope but don't initialize yet
		j := 0
		for i, varDecl := range body.Scope.VarDecls {
			if hoist[i] {
				varDecl.TokenType = js.ErrorToken
				for _, item := range varDecl.List {
					refs := bindingVars(item.Binding)
					bindingElements := make([]js.BindingElement, 0, len(refs))
				DeclaredLoop:
					for _, ref := range refs {
						for _, v := range orig {
							if ref == v {
								continue DeclaredLoop
							}
						}
						bindingElements = append(bindingElements, js.BindingElement{Binding: ref, Default: nil})
						orig = append(orig, ref)

						s := decl.Scope
						for s != nil && s != s.Func {
							s.AddUndeclared(ref)
							s = s.Parent
						}
						if item.Default != nil {
							ref.Uses++
						}
					}
					if i < best {
						// prepend
						decl.List = append(decl.List[:j], append(bindingElements, decl.List[j:]...)...)
						j += len(bindingElements)
					} else {
						// append
						decl.List = append(decl.List, bindingElements...)
					}
				}
			}
		}

		// rearrange to put array/object first
		var prevRefs []*js.Var
	BeginArrayObject:
		for i, item := range decl.List {
			refs := bindingVars(item.Binding)
			if _, ok := item.Binding.(*js.Var); !ok {
				if i != 0 {
					interferes := false
					if item.Default != nil {
					InterferenceLoop:
						for _, ref := range refs {
							for _, v := range prevRefs {
								if ref == v {
									interferes = true
									break InterferenceLoop
								}
							}
						}
					}
					if !interferes {
						decl.List[0], decl.List[i] = decl.List[i], decl.List[0]
						break BeginArrayObject
					}
				} else {
					break BeginArrayObject
				}
			}
			if item.Default != nil {
				prevRefs = append(prevRefs, refs...)
			}
		}
	}
}