From 29ca69e874ce5e8a60fa37bcf6672b7f9be1fa63 Mon Sep 17 00:00:00 2001
From: Jeff Schiller <codedread@gmail.com>
Date: Thu, 18 Jan 2024 21:00:11 -0800
Subject: [PATCH] Move EXIF profile extraction from jpeg.js into exif.js

---
 image/parsers/exif.js | 72 +++++++++++++++++++++++++++++++++++++++++++
 image/parsers/jpeg.js | 69 ++---------------------------------------
 2 files changed, 75 insertions(+), 66 deletions(-)

diff --git a/image/parsers/exif.js b/image/parsers/exif.js
index 0719dea..73a56ad 100644
--- a/image/parsers/exif.js
+++ b/image/parsers/exif.js
@@ -223,6 +223,78 @@ export function getExifValue(stream, lookAheadStream, DEBUG = false) {
   return exifValue;
 }
 
+/**
+ * Reads an Image File Directory from stream, populating the map.
+ * @param {ByteStream} stream The stream to extract the Exif value descriptors.
+ * @param {ByteStream} lookAheadStream The lookahead stream if the offset is used.
+ * @param {Map<number, ExifValue} exifValueMap This map to add the Exif values.
+ * @returns {number} The next IFD offset.
+ */
+function getExifIfd(stream, lookAheadStream, exifValueMap) {
+  let exifOffsetStream;
+  const numDirectoryEntries = stream.readNumber(2);
+  for (let entry = 0; entry < numDirectoryEntries; ++entry) {
+    const exifValue = getExifValue(stream, lookAheadStream);
+    const exifTagNumber = exifValue.tagNumber;
+    exifValueMap.set(exifTagNumber, exifValue);
+    if (exifValue.tagNumber === ExifTagNumber.EXIF_OFFSET) {
+      exifOffsetStream = lookAheadStream.tee().skip(exifValue.numericalValue);
+    }
+  } // Loop over Directory Entries.
+
+  if (exifOffsetStream) {
+    getExifIfd(exifOffsetStream, lookAheadStream, exifValueMap);
+  }
+
+  const nextIfdOffset = stream.readNumber(4);
+  return nextIfdOffset;
+}
+
+/**
+ * Reads the entire EXIF profile. The first 2 bytes in the stream must be the TIFF marker (II/MM).
+ * @param {ByteStream} stream
+ * @returns {Map<number, ExifValue} A map of all EXIF values found. The key is the EXIF tag number.
+ */
+export function getExifProfile(stream) {
+  const lookAheadStream = stream.tee();
+  const tiffByteAlign = stream.readString(2);
+  if (tiffByteAlign === 'II') {
+    stream.setLittleEndian();
+    lookAheadStream.setLittleEndian();
+  } else if (tiffByteAlign === 'MM') {
+    stream.setBigEndian();
+    lookAheadStream.setBigEndian();
+  } else {
+    throw `Invalid TIFF byte align symbol: ${tiffByteAlign}`;
+  }
+
+  const tiffMarker = stream.readNumber(2);
+  if (tiffMarker !== 0x002A) {
+    throw `Invalid marker, not 0x002a: 0x${tiffMarker.toString(16)}`;
+  }
+
+  /** @type {Map<number, ExifValue} */
+  const exifValueMap = new Map();
+
+  // The offset includes the tiffByteAlign (2), marker (2), and the offset field itself (4).
+  // It is usually 0x00000008, which means ifdOffsetSkip will almost always be zero.
+  const ifdOffsetSkip = stream.readNumber(4) - 8;
+
+  let ifdStream = stream.skip(ifdOffsetSkip).tee();
+  let nextIfdOffset;
+  while (true) {
+    nextIfdOffset = getExifIfd(ifdStream, lookAheadStream, exifValueMap);
+
+    // No more IFDs, so stop the loop.
+    if (nextIfdOffset === 0) break;
+
+    // Else, we have another IFD to read, point the stream at it.
+    ifdStream = lookAheadStream.tee().skip(nextIfdOffset);
+  }
+
+  return exifValueMap;
+}
+
 /**
  * @param {Object} obj A numeric enum.
  * @param {number} valToFind The value to find.
diff --git a/image/parsers/jpeg.js b/image/parsers/jpeg.js
index 4b16c7b..fdf1d0b 100644
--- a/image/parsers/jpeg.js
+++ b/image/parsers/jpeg.js
@@ -9,11 +9,9 @@
  */
 
 import { ByteStream } from '../../io/bytestream.js';
-import { ExifTagNumber, getExifValue } from './exif.js';
+import { getExifProfile } from './exif.js';
 
-/**
- * @typedef {import('./exif.js').ExifValue} ExifValue
- */
+/** @typedef {import('./exif.js').ExifValue} ExifValue */
 
 // https://en.wikipedia.org/wiki/JPEG_File_Interchange_Format
 // https://www.media.mit.edu/pia/Research/deepview/exif.html
@@ -388,41 +386,7 @@ export class JpegParser extends EventTarget {
         }
         if (this.bstream.readNumber(2) !== 0) throw `No null byte termination`;
 
-        const lookAheadStream = this.bstream.tee();
-        const tiffByteAlign = this.bstream.readString(2);
-        if (tiffByteAlign === 'II') {
-          this.bstream.setLittleEndian();
-          lookAheadStream.setLittleEndian();
-        } else if (tiffByteAlign === 'MM') {
-          this.bstream.setBigEndian();
-          lookAheadStream.setBigEndian();
-        } else {
-          throw `Invalid TIFF byte align symbol: ${tiffByteAlign}`;
-        }
-
-        const tiffMarker = this.bstream.readNumber(2);
-        if (tiffMarker !== 0x002A) {
-          throw `Invalid marker, not 0x002a: 0x${tiffMarker.toString(16)}`;
-        }
-
-        /** @type {Map<number, ExifValue} */
-        const exifValueMap = new Map();
-
-        // The offset includes the tiffByteAlign (2), marker (2), and the offset field itself (4).
-        const ifdOffset = this.bstream.readNumber(4) - 8;
-
-        let ifdStream = this.bstream.tee();
-        let nextIfdOffset;
-        while (true) {
-          nextIfdOffset = this.readExifIfd(ifdStream, lookAheadStream, exifValueMap);
-
-          // No more IFDs, so stop the loop.
-          if (nextIfdOffset === 0) break;
-
-          // Else, we have another IFD to read, point the stream at it.
-          ifdStream = lookAheadStream.tee().skip(nextIfdOffset);
-        }
-
+        const exifValueMap = getExifProfile(this.bstream);
         this.dispatchEvent(new JpegApp1ExifEvent(exifValueMap));
 
         this.bstream = skipAheadStream;
@@ -582,31 +546,4 @@ export class JpegParser extends EventTarget {
       }
     } while (jpegMarker !== JpegSegmentType.EOI);
   }
-
-  /**
-   * Reads an Image File Directory from stream.
-   * @param {ByteStream} stream The stream to extract the Exif value descriptor.
-   * @param {ByteStream} lookAheadStream The lookahead stream if the offset is used.
-   * @param {Map<number, ExifValue} exifValueMap This map to add the Exif values.
-   * @returns {number} The next IFD offset.
-   */
-  readExifIfd(stream, lookAheadStream, exifValueMap) {
-    let exifOffsetStream;
-    const numDirectoryEntries = stream.readNumber(2);
-    for (let entry = 0; entry < numDirectoryEntries; ++entry) {
-      const exifValue = getExifValue(stream, lookAheadStream, DEBUG);
-      const exifTagNumber = exifValue.tagNumber;
-      exifValueMap.set(exifTagNumber, exifValue);
-      if (exifValue.tagNumber === ExifTagNumber.EXIF_OFFSET) {
-        exifOffsetStream = lookAheadStream.tee().skip(exifValue.numericalValue);
-      }
-    } // Loop over Directory Entries.
-
-    if (exifOffsetStream) {
-      this.readExifIfd(exifOffsetStream, lookAheadStream, exifValueMap);
-    }
-
-    const nextIfdOffset = stream.readNumber(4);
-    return nextIfdOffset;
-  }
 }