From ce5bf71f6b13f5cf38de1c3f0d3e8363e1716d18 Mon Sep 17 00:00:00 2001
From: bel <bel@bel.bel>
Date: Sun, 29 Oct 2023 09:05:39 -0600
Subject: [PATCH] wip

---
 src/ledger/file.go      | 37 +++++++++++++++++++
 src/ledger/file_test.go | 82 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 119 insertions(+)

diff --git a/src/ledger/file.go b/src/ledger/file.go
index 30ecb24..57abd1a 100644
--- a/src/ledger/file.go
+++ b/src/ledger/file.go
@@ -1,6 +1,8 @@
 package ledger
 
 import (
+	"bufio"
+	"bytes"
 	"fmt"
 	"io"
 	"io/fs"
@@ -20,6 +22,41 @@ func NewFiles(p string, q ...string) (Files, error) {
 	return f, err
 }
 
+func (files Files) TempGetLastNLines(n int) ([]string, error) {
+	p := files.paths()[0]
+	f, err := os.Open(p)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	return peekLastNLines(io.Discard, bufio.NewReader(f), n)
+}
+
+func (files Files) TempSetLastNLines(n int, lines []string) error {
+	panic("e")
+}
+
+func peekLastNLines(w io.Writer, r *bufio.Reader, n int) ([]string, error) {
+	lastNLines := make([]string, 0, n)
+	for {
+		line, err := r.ReadBytes('\n')
+		if len(line) > 0 {
+			lastNLines = append(lastNLines, string(bytes.TrimRight(line, "\n")))
+			for i := 0; i < len(lastNLines)-n; i++ {
+				fmt.Fprintln(w, lastNLines[i])
+			}
+			lastNLines = lastNLines[max(0, len(lastNLines)-n):]
+		}
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			return nil, err
+		}
+	}
+	return lastNLines, nil
+}
+
 func (files Files) paths() []string {
 	result := make([]string, 0, len(files))
 	for i := range files {
diff --git a/src/ledger/file_test.go b/src/ledger/file_test.go
index fb93259..9b8b356 100644
--- a/src/ledger/file_test.go
+++ b/src/ledger/file_test.go
@@ -1,6 +1,8 @@
 package ledger
 
 import (
+	"encoding/base64"
+	"fmt"
 	"os"
 	"path"
 	"path/filepath"
@@ -290,3 +292,83 @@ func TestFilesOfDir(t *testing.T) {
 		t.Error(paths)
 	}
 }
+
+func TestFilesTempGetLastNLines(t *testing.T) {
+	cases := map[string]struct {
+		input string
+		n     int
+		want  []string
+	}{
+		"empty": {},
+		"get n lines from empty file": {
+			input: "",
+			n:     5,
+			want:  []string{},
+		},
+		"get 0 lines from file": {
+			input: "#a\n#b",
+			n:     0,
+			want:  []string{},
+		},
+		"get 3 lines from 2 line file": {
+			input: "#a\n#b",
+			n:     3,
+			want:  []string{"#a", "#b"},
+		},
+		"get 2 lines from 3 line file": {
+			input: "#a\n#b\n#c",
+			n:     2,
+			want:  []string{"#b", "#c"},
+		},
+	}
+
+	for name, d := range cases {
+		c := d
+		t.Run(name, func(t *testing.T) {
+			p := path.Join(t.TempDir(), base64.URLEncoding.EncodeToString([]byte(t.Name())))
+			os.WriteFile(p, []byte(c.input), os.ModePerm)
+			files, err := NewFiles(p)
+			if err != nil {
+				panic(err)
+			}
+			if got, err := files.TempGetLastNLines(c.n); err != nil {
+				t.Fatal(err)
+			} else if fmt.Sprint(got) != fmt.Sprint(c.want) {
+				for i := range c.want {
+					t.Logf("want[%d] = %q", i, c.want[i])
+				}
+				for i := range got {
+					t.Logf(" got[%d] = %q", i, got[i])
+				}
+				t.Errorf("wanted\n\t%+v,  got\n\t%+v", c.want, got)
+			}
+		})
+	}
+}
+
+func TestFilesTempSetLastNLines(t *testing.T) {
+	cases := map[string]struct {
+		given string
+		input []string
+		n     int
+		want  string
+	}{
+		"empty": {},
+	}
+
+	for name, d := range cases {
+		c := d
+		t.Run(name, func(t *testing.T) {
+			p := path.Join(t.TempDir(), base64.URLEncoding.EncodeToString([]byte(t.Name())))
+			os.WriteFile(p, []byte(c.given), os.ModePerm)
+			files := Files([]string{p})
+			if err := files.TempSetLastNLines(c.n, c.input); err != nil {
+				t.Fatal(err)
+			}
+			got, _ := os.ReadFile(p)
+			if string(got) != c.want {
+				t.Errorf("want\n\t%s, got\n\t%s", c.want, got)
+			}
+		})
+	}
+}