package funcs

import (
	"encoding/base64"
	"fmt"
	"io/ioutil"
	"os"
	"path/filepath"
	"unicode/utf8"

	"github.com/hashicorp/hcl2/hcl"
	"github.com/hashicorp/hcl2/hcl/hclsyntax"
	homedir "github.com/mitchellh/go-homedir"
	"github.com/zclconf/go-cty/cty"
	"github.com/zclconf/go-cty/cty/function"
)

// MakeFileFunc constructs a function that takes a file path and returns the
// contents of that file, either directly as a string (where valid UTF-8 is
// required) or as a string containing base64 bytes.
func MakeFileFunc(baseDir string, encBase64 bool) function.Function {
	return function.New(&function.Spec{
		Params: []function.Parameter{
			{
				Name: "path",
				Type: cty.String,
			},
		},
		Type: function.StaticReturnType(cty.String),
		Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
			path := args[0].AsString()
			src, err := readFileBytes(baseDir, path)
			if err != nil {
				return cty.UnknownVal(cty.String), err
			}

			switch {
			case encBase64:
				enc := base64.StdEncoding.EncodeToString(src)
				return cty.StringVal(enc), nil
			default:
				if !utf8.Valid(src) {
					return cty.UnknownVal(cty.String), fmt.Errorf("contents of %s are not valid UTF-8; to read arbitrary bytes, use the filebase64 function instead", path)
				}
				return cty.StringVal(string(src)), nil
			}
		},
	})
}

// MakeTemplateFileFunc constructs a function that takes a file path and
// an arbitrary object of named values and attempts to render the referenced
// file as a template using HCL template syntax.
//
// The template itself may recursively call other functions so a callback
// must be provided to get access to those functions. The template cannot,
// however, access any variables defined in the scope: it is restricted only to
// those variables provided in the second function argument, to ensure that all
// dependencies on other graph nodes can be seen before executing this function.
//
// As a special exception, a referenced template file may not recursively call
// the templatefile function, since that would risk the same file being
// included into itself indefinitely.
func MakeTemplateFileFunc(baseDir string, funcsCb func() map[string]function.Function) function.Function {

	params := []function.Parameter{
		{
			Name: "path",
			Type: cty.String,
		},
		{
			Name: "vars",
			Type: cty.DynamicPseudoType,
		},
	}

	loadTmpl := func(fn string) (hcl.Expression, error) {
		// We re-use File here to ensure the same filename interpretation
		// as it does, along with its other safety checks.
		tmplVal, err := File(baseDir, cty.StringVal(fn))
		if err != nil {
			return nil, err
		}

		expr, diags := hclsyntax.ParseTemplate([]byte(tmplVal.AsString()), fn, hcl.Pos{Line: 1, Column: 1})
		if diags.HasErrors() {
			return nil, diags
		}

		return expr, nil
	}

	renderTmpl := func(expr hcl.Expression, varsVal cty.Value) (cty.Value, error) {
		if varsTy := varsVal.Type(); !(varsTy.IsMapType() || varsTy.IsObjectType()) {
			return cty.DynamicVal, function.NewArgErrorf(2, "invalid vars value: must be a map") // or an object, but we don't strongly distinguish these most of the time
		}

		ctx := &hcl.EvalContext{
			Variables: varsVal.AsValueMap(),
		}

		// We'll pre-check references in the template here so we can give a
		// more specialized error message than HCL would by default, so it's
		// clearer that this problem is coming from a templatefile call.
		for _, traversal := range expr.Variables() {
			root := traversal.RootName()
			if _, ok := ctx.Variables[root]; !ok {
				return cty.DynamicVal, function.NewArgErrorf(2, "vars map does not contain key %q, referenced at %s", root, traversal[0].SourceRange())
			}
		}

		givenFuncs := funcsCb() // this callback indirection is to avoid chicken/egg problems
		funcs := make(map[string]function.Function, len(givenFuncs))
		for name, fn := range givenFuncs {
			if name == "templatefile" {
				// We stub this one out to prevent recursive calls.
				funcs[name] = function.New(&function.Spec{
					Params: params,
					Type: func(args []cty.Value) (cty.Type, error) {
						return cty.NilType, fmt.Errorf("cannot recursively call templatefile from inside templatefile call")
					},
				})
				continue
			}
			funcs[name] = fn
		}
		ctx.Functions = funcs

		val, diags := expr.Value(ctx)
		if diags.HasErrors() {
			return cty.DynamicVal, diags
		}
		return val, nil
	}

	return function.New(&function.Spec{
		Params: params,
		Type: func(args []cty.Value) (cty.Type, error) {
			if !(args[0].IsKnown() && args[1].IsKnown()) {
				return cty.DynamicPseudoType, nil
			}

			// We'll render our template now to see what result type it produces.
			// A template consisting only of a single interpolation an potentially
			// return any type.
			expr, err := loadTmpl(args[0].AsString())
			if err != nil {
				return cty.DynamicPseudoType, err
			}

			// This is safe even if args[1] contains unknowns because the HCL
			// template renderer itself knows how to short-circuit those.
			val, err := renderTmpl(expr, args[1])
			return val.Type(), err
		},
		Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
			expr, err := loadTmpl(args[0].AsString())
			if err != nil {
				return cty.DynamicVal, err
			}
			return renderTmpl(expr, args[1])
		},
	})

}

// MakeFileExistsFunc constructs a function that takes a path
// and determines whether a file exists at that path
func MakeFileExistsFunc(baseDir string) function.Function {
	return function.New(&function.Spec{
		Params: []function.Parameter{
			{
				Name: "path",
				Type: cty.String,
			},
		},
		Type: function.StaticReturnType(cty.Bool),
		Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
			path := args[0].AsString()
			path, err := homedir.Expand(path)
			if err != nil {
				return cty.UnknownVal(cty.Bool), fmt.Errorf("failed to expand ~: %s", err)
			}

			if !filepath.IsAbs(path) {
				path = filepath.Join(baseDir, path)
			}

			// Ensure that the path is canonical for the host OS
			path = filepath.Clean(path)

			fi, err := os.Stat(path)
			if err != nil {
				if os.IsNotExist(err) {
					return cty.False, nil
				}
				return cty.UnknownVal(cty.Bool), fmt.Errorf("failed to stat %s", path)
			}

			if fi.Mode().IsRegular() {
				return cty.True, nil
			}

			return cty.False, fmt.Errorf("%s is not a regular file, but %q",
				path, fi.Mode().String())
		},
	})
}

// BasenameFunc constructs a function that takes a string containing a filesystem path
// and removes all except the last portion from it.
var BasenameFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name: "path",
			Type: cty.String,
		},
	},
	Type: function.StaticReturnType(cty.String),
	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
		return cty.StringVal(filepath.Base(args[0].AsString())), nil
	},
})

// DirnameFunc constructs a function that takes a string containing a filesystem path
// and removes the last portion from it.
var DirnameFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name: "path",
			Type: cty.String,
		},
	},
	Type: function.StaticReturnType(cty.String),
	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
		return cty.StringVal(filepath.Dir(args[0].AsString())), nil
	},
})

// PathExpandFunc constructs a function that expands a leading ~ character to the current user's home directory.
var PathExpandFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name: "path",
			Type: cty.String,
		},
	},
	Type: function.StaticReturnType(cty.String),
	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {

		homePath, err := homedir.Expand(args[0].AsString())
		return cty.StringVal(homePath), err
	},
})

func readFileBytes(baseDir, path string) ([]byte, error) {
	path, err := homedir.Expand(path)
	if err != nil {
		return nil, fmt.Errorf("failed to expand ~: %s", err)
	}

	if !filepath.IsAbs(path) {
		path = filepath.Join(baseDir, path)
	}

	// Ensure that the path is canonical for the host OS
	path = filepath.Clean(path)

	src, err := ioutil.ReadFile(path)
	if err != nil {
		// ReadFile does not return Terraform-user-friendly error
		// messages, so we'll provide our own.
		if os.IsNotExist(err) {
			return nil, fmt.Errorf("no file exists at %s", path)
		}
		return nil, fmt.Errorf("failed to read %s", path)
	}

	return src, nil
}

// File reads the contents of the file at the given path.
//
// The file must contain valid UTF-8 bytes, or this function will return an error.
//
// The underlying function implementation works relative to a particular base
// directory, so this wrapper takes a base directory string and uses it to
// construct the underlying function before calling it.
func File(baseDir string, path cty.Value) (cty.Value, error) {
	fn := MakeFileFunc(baseDir, false)
	return fn.Call([]cty.Value{path})
}

// FileExists determines whether a file exists at the given path.
//
// The underlying function implementation works relative to a particular base
// directory, so this wrapper takes a base directory string and uses it to
// construct the underlying function before calling it.
func FileExists(baseDir string, path cty.Value) (cty.Value, error) {
	fn := MakeFileExistsFunc(baseDir)
	return fn.Call([]cty.Value{path})
}

// FileBase64 reads the contents of the file at the given path.
//
// The bytes from the file are encoded as base64 before returning.
//
// The underlying function implementation works relative to a particular base
// directory, so this wrapper takes a base directory string and uses it to
// construct the underlying function before calling it.
func FileBase64(baseDir string, path cty.Value) (cty.Value, error) {
	fn := MakeFileFunc(baseDir, true)
	return fn.Call([]cty.Value{path})
}

// Basename takes a string containing a filesystem path and removes all except the last portion from it.
//
// The underlying function implementation works only with the path string and does not access the filesystem itself.
// It is therefore unable to take into account filesystem features such as symlinks.
//
// If the path is empty then the result is ".", representing the current working directory.
func Basename(path cty.Value) (cty.Value, error) {
	return BasenameFunc.Call([]cty.Value{path})
}

// Dirname takes a string containing a filesystem path and removes the last portion from it.
//
// The underlying function implementation works only with the path string and does not access the filesystem itself.
// It is therefore unable to take into account filesystem features such as symlinks.
//
// If the path is empty then the result is ".", representing the current working directory.
func Dirname(path cty.Value) (cty.Value, error) {
	return DirnameFunc.Call([]cty.Value{path})
}

// Pathexpand takes a string that might begin with a `~` segment, and if so it replaces that segment with
// the current user's home directory path.
//
// The underlying function implementation works only with the path string and does not access the filesystem itself.
// It is therefore unable to take into account filesystem features such as symlinks.
//
// If the leading segment in the path is not `~` then the given path is returned unmodified.
func Pathexpand(path cty.Value) (cty.Value, error) {
	return PathExpandFunc.Call([]cty.Value{path})
}