// Package docgen turns the configuration structs into reference documentation.
//
// Generated rather than written, because a hand-written reference is a promise
// that someone will remember. This project added six settings in a fortnight —
// store, servers, lease_ttl, thread and memory retention, min_disk_gb — and
// every one of them reached an operator only through a narrative document that
// happened to mention it. A reference nobody maintains is worse than none: it
// is the same list, quietly missing the setting you came to look up.
//
// Derived from the source, so the only way for it to be wrong is for the code
// to be wrong. A test regenerates it and fails when it has drifted, which is
// what keeps that true after everyone has forgotten this exists.
package docgen

//go:generate go run ./gen

import (
	"fmt"
	"go/ast"
	"go/parser"
	"go/token"
	"reflect"
	"sort"
	"strings"
)

// Field is one configuration key.
type Field struct {
	Key  string
	Type string
	Doc  string
	// Nested names the struct this key expands into, by its ID
	// (config.Auth), empty for a leaf. It is what lets the reference say
	// "see Auth" rather than printing the same twenty keys under four
	// parents.
	Nested string
}

// Block is a struct that appears in configuration.
type Block struct {
	// Package and Name are where the struct is declared: what tells
	// tlsconf.Config from the agent's Config.
	Package string
	Name    string
	// Title is what it is called in the reference and on the screens: the
	// name alone where no other block shares it, the package's name with it
	// where one does.
	Title  string
	Doc    string
	Fields []Field
}

// ID is the block's package and name, unique where Name is not.
func (b Block) ID() string { return b.Package + "." + b.Name }

// Collect reads koanf-tagged structs out of Go source files.
func Collect(files []string) ([]Block, error) {
	fset := token.NewFileSet()
	var blocks []Block
	seen := map[string]bool{}

	for _, path := range files {
		f, err := parser.ParseFile(fset, path, nil, parser.ParseComments)
		if err != nil {
			return nil, fmt.Errorf("parsing %s: %w", path, err)
		}
		ast.Inspect(f, func(n ast.Node) bool {
			ts, ok := n.(*ast.TypeSpec)
			if !ok {
				return true
			}
			st, ok := ts.Type.(*ast.StructType)
			if !ok {
				return true
			}
			b, ok := blockOf(f.Name.Name, ts, st)
			if !ok || seen[b.ID()] {
				return true
			}
			seen[b.ID()] = true
			blocks = append(blocks, b)
			return true
		})
	}
	// Named by their own name unless another block has it too. Keyed by
	// name alone, tlsconf.Config, engines.Config and supervisor.Config were
	// each taken for the agent's Config and dropped: the reference had no
	// TLS settings at all.
	count := map[string]int{}
	for _, b := range blocks {
		count[b.Name]++
	}
	for i := range blocks {
		blocks[i].Title = blocks[i].Name
		if count[blocks[i].Name] > 1 {
			blocks[i].Title = blocks[i].ID()
		}
	}
	sort.Slice(blocks, func(i, j int) bool { return blocks[i].Title < blocks[j].Title })
	return blocks, nil
}

// blockOf builds a Block, or reports that this struct carries no settings.
func blockOf(pkg string, ts *ast.TypeSpec, st *ast.StructType) (Block, bool) {
	b := Block{Package: pkg, Name: ts.Name.Name, Doc: docText(ts.Doc)}
	for _, f := range st.Fields.List {
		if f.Tag == nil {
			continue
		}
		tag := reflect.StructTag(strings.Trim(f.Tag.Value, "`"))
		key, ok := tag.Lookup("koanf")
		// "-" is a field carried in the struct but never read from a file;
		// listing it would send someone looking for a setting that does not
		// exist.
		if !ok || key == "" || key == "-" {
			continue
		}
		typ, nested := typeOf(pkg, f.Type)
		b.Fields = append(b.Fields, Field{
			Key: key, Type: typ, Doc: docText(f.Doc), Nested: nested,
		})
	}
	return b, len(b.Fields) > 0
}

// typeOf renders a field's type, and names the struct it expands into.
func typeOf(pkg string, e ast.Expr) (rendered, nested string) {
	switch t := e.(type) {
	case *ast.Ident:
		if isNamedStruct(t.Name) {
			return t.Name, pkg + "." + t.Name
		}
		return t.Name, ""
	case *ast.SelectorExpr:
		// A type from another package, such as tlsconf.Config. Standard
		// library types are values, not blocks: a time.Duration is a
		// setting, written "30s".
		name := t.Sel.Name
		if x, ok := t.X.(*ast.Ident); ok {
			if x.Name == "time" {
				return x.Name + "." + name, ""
			}
			return x.Name + "." + name, x.Name + "." + name
		}
		return name, ""
	case *ast.ArrayType:
		inner, _ := typeOf(pkg, t.Elt)
		return "[]" + inner, ""
	case *ast.MapType:
		k, _ := typeOf(pkg, t.Key)
		v, _ := typeOf(pkg, t.Value)
		return "map[" + k + "]" + v, ""
	case *ast.StarExpr:
		inner, n := typeOf(pkg, t.X)
		return "*" + inner, n
	default:
		return "value", ""
	}
}

// isNamedStruct guesses whether an identifier names a configuration block.
//
// By capitalisation, because the alternative is resolving types across
// packages for a document. A wrong guess costs a "see also" that points at
// nothing, which the renderer drops.
func isNamedStruct(name string) bool {
	if name == "" {
		return false
	}
	switch name {
	case "string", "bool", "int", "int64", "float64", "any", "byte", "rune":
		return false
	}
	return name[0] >= 'A' && name[0] <= 'Z'
}

// docText flattens a comment group into one paragraph per blank line.
func docText(g *ast.CommentGroup) string {
	if g == nil {
		return ""
	}
	var lines []string
	for _, c := range g.List {
		t := strings.TrimPrefix(c.Text, "//")
		lines = append(lines, strings.TrimPrefix(t, " "))
	}
	return strings.TrimSpace(strings.Join(lines, "\n"))
}