From 17fb6add26291b31f7020e3551a7c8487130a747 Mon Sep 17 00:00:00 2001
From: "F.O." <scrotadamus@insiberia.net>
Date: Sun, 16 Feb 2025 17:56:08 +0100
Subject: genesi

---
 document/hash.go | 104 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 104 insertions(+)
 create mode 100644 document/hash.go

(limited to 'document/hash.go')

diff --git a/document/hash.go b/document/hash.go
new file mode 100644
index 0000000..fe9520c
--- /dev/null
+++ b/document/hash.go
@@ -0,0 +1,104 @@
+package document
+
+import (
+	"fmt"
+	"os"
+
+	"unsafe"
+	"reflect"
+
+	"math"
+
+	"runtime"
+	"sync"
+
+	"crypto/hmac"
+	"crypto/sha256"
+
+	"github.com/scrotadamus/ghligh/go-poppler"
+)
+
+
+var ghlighKey = []byte("ghligh-pdf-doc")
+
+var bufPool = sync.Pool{
+	New: func() interface{} {
+		return make([]byte, 0, os.Getpagesize())
+	},
+}
+
+type pageResult struct {
+	index int
+	buf   []byte
+}
+
+
+func sqrtInt(n int) int{
+	return int(math.Sqrt(float64(n)))
+}
+
+func continueAt(i, n int) bool {
+	// Very unlikely to edit a pdf and add a page in the center
+	return i < sqrtInt(n)/2  || i > n - sqrtInt(n)/2
+}
+
+// generate identifier from document based on document text (use layout instead)
+func (d *GhlighDoc) HashDoc() string {
+	nPages := d.doc.GetNPages()
+
+	hmacHash := hmac.New(sha256.New, ghlighKey)
+	resultsCh := make(chan pageResult, nPages)
+
+	var wg sync.WaitGroup
+
+	maxWorkers := runtime.NumCPU() + 1
+	sem := make(chan struct{}, maxWorkers)
+
+
+	for i := 0; continueAt(i, nPages); i++ {
+		wg.Add(1)
+		sem <- struct{}{}
+		go func(i int) {
+			defer wg.Done()
+			defer func() { <-sem }()
+
+			page := d.doc.GetPage(i)
+			text := page.Text()
+			page.Close()
+
+			buf := bufPool.Get().([]byte)
+			buf = buf[:0]
+			buf = append(buf, []byte(text)...)
+
+			resultsCh <- pageResult{index: i, buf: buf}
+		}(i)
+	}
+
+	wg.Wait()
+	close(resultsCh)
+
+	results := make([][]byte, nPages)
+	for res := range resultsCh {
+		results[res.index] = res.buf
+	}
+
+	for i := 0; continueAt(i, nPages); i++ {
+		hmacHash.Write(results[i])
+		hmacHash.Write([]byte{byte(i)})
+		bufPool.Put(results[i])
+	}
+	hmacHash.Write([]byte{byte(nPages)})
+
+	return fmt.Sprintf("%x", hmacHash.Sum(nil))
+}
+
+func rectToBytes(r *poppler.Rectangle) []byte {
+	size := int(unsafe.Sizeof(*r))
+
+	var sliceHeader reflect.SliceHeader
+	sliceHeader.Data = uintptr(unsafe.Pointer(r))
+	sliceHeader.Len = size
+	sliceHeader.Cap = size
+
+	return *(*[]byte)(unsafe.Pointer(&sliceHeader))
+}
-- 
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