// Package httpstatus reports discouraged HTTP status-code usage.
package httpstatus

import (
	"fmt"
	"go/ast"
	"go/constant"
	"go/types"
	"strconv"
	"strings"

	"golang.org/x/tools/go/analysis"
	"golang.org/x/tools/go/analysis/passes/inspect"
	"golang.org/x/tools/go/ast/inspector"
)

const netHTTPPath = "net/http"

var defaultTypicalStatusCodes = []int{
	200, 204,
	302,
	400, 401, 403, 404, 429,
	502, 504,
}

var Analyzer = NewAnalyzer(Settings{})

func NewAnalyzer(settings Settings) *analysis.Analyzer {
	if settings.TypicalStatusCodes == nil {
		settings.TypicalStatusCodes = defaultTypicalStatusCodes
	}

	typicalStatusCodes := make(map[int]struct{}, len(settings.TypicalStatusCodes))
	for _, code := range settings.TypicalStatusCodes {
		typicalStatusCodes[code] = struct{}{}
	}

	return &analysis.Analyzer{
		Name:     "httpstatus",
		Doc:      "checks HTTP status-code usage",
		Requires: []*analysis.Analyzer{inspect.Analyzer},
		Run: func(pass *analysis.Pass) (any, error) {
			insp, ok := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
			if !ok {
				panic("inspect.Analyzer did not produce *inspector.Inspector")
			}

			// ---------------
			// Checks
			// ---------------

			// Disallows `http.StatusBlahBlah` symbols for typical status codes
			enforceIntegerLiterals := func(node *ast.SelectorExpr) {
				status, ok := pass.TypesInfo.Uses[node.Sel].(*types.Const)
				if !ok || status.Pkg() == nil || status.Pkg().Path() != netHTTPPath || !strings.HasPrefix(status.Name(), "Status") {
					return
				}
				// Check if it corresponds to one of the codes in the "typical" list
				code, ok := integerConstant(status.Val())
				if !ok {
					return
				}
				if _, ok := typicalStatusCodes[code]; !ok {
					return
				}

				pass.Report(analysis.Diagnostic{
					Pos:     node.Pos(),
					End:     node.End(),
					Message: fmt.Sprintf("HTTP status code %d should use its integer literal", code),
					SuggestedFixes: []analysis.SuggestedFix{{
						Message: "Use the integer literal",
						TextEdits: []analysis.TextEdit{{
							Pos:     node.Pos(),
							End:     node.End(),
							NewText: []byte(strconv.Itoa(code)),
						}},
					}},
				})
			}

			// Disallows sending non-typical response statuses
			requireTypicalResponseStatus := func(node *ast.CallExpr) {
				checkArgument := func(argIdx int) {
					// Require a status-code argument.
					if argIdx < 0 || len(node.Args) <= argIdx {
						return
					}

					// Report non-typical codes.
					code, ok := integerConstant(pass.TypesInfo.Types[node.Args[argIdx]].Value)
					if !ok {
						return
					}
					if _, ok := typicalStatusCodes[code]; !ok {
						pass.Reportf(node.Args[argIdx].Pos(), "non-typical HTTP status code %d is not allowed", code)
					}
				}
				switch functionExpression := node.Fun.(type) {
				case *ast.Ident:
					// Handle dot-imported http.Error.
					function, ok := pass.TypesInfo.Uses[functionExpression].(*types.Func)
					if ok && function.Pkg() != nil && function.Pkg().Path() == netHTTPPath && function.Name() == "Error" {
						checkArgument(2)
					}
				case *ast.SelectorExpr:
					// Handle qualified http.Error.
					function, _ := pass.TypesInfo.Uses[functionExpression.Sel].(*types.Func)
					if function != nil && function.Pkg() != nil && function.Pkg().Path() == netHTTPPath && function.Name() == "Error" {
						checkArgument(2)
						return
					}

					// Handle both forms of WriteHeader.
					selection := pass.TypesInfo.Selections[functionExpression]
					if selection == nil {
						return
					}
					method, ok := selection.Obj().(*types.Func)
					if ok && method.Pkg() != nil && method.Pkg().Path() == netHTTPPath && method.Name() == "WriteHeader" {
						if selection.Kind() == types.MethodExpr {
							checkArgument(1) // `http.ResponseWriter.WriteHeader`
						} else {
							checkArgument(0) // `w.WriteHeader`
						}
					}
				default:
					return
				}
			}

			// -------------------
			// Walk the tree
			// -------------------

			insp.Preorder([]ast.Node{(*ast.SelectorExpr)(nil), (*ast.CallExpr)(nil)}, func(node ast.Node) {
				switch node := node.(type) {
				case *ast.SelectorExpr:
					enforceIntegerLiterals(node)
				case *ast.CallExpr:
					if !settings.IsRestricted {
						return
					}
					requireTypicalResponseStatus(node)
				default:
					panic("inspector passed an unexpected node")
				}
			})

			return nil, nil
		},
	}
}

// integerConstant attempts to extract the value of a constant.Value as an int.
func integerConstant(value constant.Value) (int, bool) {
	if value == nil {
		return 0, false
	}

	integer, ok := constant.Int64Val(value)
	if !ok || int64(int(integer)) != integer {
		return 0, false
	}

	return int(integer), true
}