// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package source

import (
	"fmt"
	"go/ast"
	"go/token"
	"go/types"
	"strings"
)

// indexExprAtPos returns the index of the expression containing pos.
func indexExprAtPos(pos token.Pos, args []ast.Expr) int {
	for i, expr := range args {
		if expr.Pos() <= pos && pos <= expr.End() {
			return i
		}
	}
	return len(args)
}

func exprAtPos(pos token.Pos, args []ast.Expr) ast.Expr {
	for _, expr := range args {
		if expr.Pos() <= pos && pos <= expr.End() {
			return expr
		}
	}
	return nil
}

// fieldSelections returns the set of fields that can
// be selected from a value of type T.
func fieldSelections(T types.Type) (fields []*types.Var) {
	// TODO(adonovan): this algorithm doesn't exclude ambiguous
	// selections that match more than one field/method.
	// types.NewSelectionSet should do that for us.

	seen := make(map[types.Type]bool) // for termination on recursive types
	var visit func(T types.Type)
	visit = func(T types.Type) {
		if !seen[T] {
			seen[T] = true
			if T, ok := deref(T).Underlying().(*types.Struct); ok {
				for i := 0; i < T.NumFields(); i++ {
					f := T.Field(i)
					fields = append(fields, f)
					if f.Anonymous() {
						visit(f.Type())
					}
				}
			}
		}
	}
	visit(T)

	return fields
}

// resolveInvalid traverses the node of the AST that defines the scope
// containing the declaration of obj, and attempts to find a user-friendly
// name for its invalid type. The resulting Object and its Type are fake.
func resolveInvalid(obj types.Object, node ast.Node, info *types.Info) types.Object {
	// Construct a fake type for the object and return a fake object with this type.
	formatResult := func(expr ast.Expr) types.Object {
		var typename string
		switch t := expr.(type) {
		case *ast.SelectorExpr:
			typename = fmt.Sprintf("%s.%s", t.X, t.Sel)
		case *ast.Ident:
			typename = t.String()
		default:
			return nil
		}
		typ := types.NewNamed(types.NewTypeName(token.NoPos, obj.Pkg(), typename, nil), nil, nil)
		return types.NewVar(obj.Pos(), obj.Pkg(), obj.Name(), typ)
	}
	var resultExpr ast.Expr
	ast.Inspect(node, func(node ast.Node) bool {
		switch n := node.(type) {
		case *ast.ValueSpec:
			for _, name := range n.Names {
				if info.Defs[name] == obj {
					resultExpr = n.Type
				}
			}
			return false
		case *ast.Field: // This case handles parameters and results of a FuncDecl or FuncLit.
			for _, name := range n.Names {
				if info.Defs[name] == obj {
					resultExpr = n.Type
				}
			}
			return false
		// TODO(rstambler): Handle range statements.
		default:
			return true
		}
	})
	return formatResult(resultExpr)
}

func lookupBuiltinDecl(v View, name string) interface{} {
	builtinPkg := v.BuiltinPackage()
	if builtinPkg == nil || builtinPkg.Scope == nil {
		return nil
	}
	obj := builtinPkg.Scope.Lookup(name)
	if obj == nil {
		return nil
	}
	return obj.Decl
}

func isPointer(T types.Type) bool {
	_, ok := T.(*types.Pointer)
	return ok
}

// deref returns a pointer's element type; otherwise it returns typ.
func deref(typ types.Type) types.Type {
	if p, ok := typ.Underlying().(*types.Pointer); ok {
		return p.Elem()
	}
	return typ
}

func formatParams(tup *types.Tuple, variadic bool, qf types.Qualifier) []string {
	params := make([]string, 0, tup.Len())
	for i := 0; i < tup.Len(); i++ {
		el := tup.At(i)
		typ := types.TypeString(el.Type(), qf)

		// Handle a variadic parameter (can only be the final parameter).
		if variadic && i == tup.Len()-1 {
			typ = strings.Replace(typ, "[]", "...", 1)
		}

		if el.Name() == "" {
			params = append(params, typ)
		} else {
			params = append(params, el.Name()+" "+typ)
		}
	}
	return params
}

func formatResults(tup *types.Tuple, qf types.Qualifier) ([]string, bool) {
	var writeResultParens bool
	results := make([]string, 0, tup.Len())
	for i := 0; i < tup.Len(); i++ {
		if i >= 1 {
			writeResultParens = true
		}
		el := tup.At(i)
		typ := types.TypeString(el.Type(), qf)

		if el.Name() == "" {
			results = append(results, typ)
		} else {
			if i == 0 {
				writeResultParens = true
			}
			results = append(results, el.Name()+" "+typ)
		}
	}
	return results, writeResultParens
}

// formatType returns the detail and kind for an object of type *types.TypeName.
func formatType(typ types.Type, qf types.Qualifier) (detail string, kind CompletionItemKind) {
	if types.IsInterface(typ) {
		detail = "interface{...}"
		kind = InterfaceCompletionItem
	} else if _, ok := typ.(*types.Struct); ok {
		detail = "struct{...}"
		kind = StructCompletionItem
	} else if typ != typ.Underlying() {
		detail, kind = formatType(typ.Underlying(), qf)
	} else {
		detail = types.TypeString(typ, qf)
		kind = TypeCompletionItem
	}
	return detail, kind
}

func formatFunction(name string, params []string, results []string, writeResultParens bool) (string, string) {
	var label, detail strings.Builder
	label.WriteString(name)
	label.WriteByte('(')
	for i, p := range params {
		if i > 0 {
			label.WriteString(", ")
		}
		label.WriteString(p)
	}
	label.WriteByte(')')

	if writeResultParens {
		detail.WriteByte('(')
	}
	for i, p := range results {
		if i > 0 {
			detail.WriteString(", ")
		}
		detail.WriteString(p)
	}
	if writeResultParens {
		detail.WriteByte(')')
	}

	return label.String(), detail.String()
}