From d75f772ffa330b28539a0c6493803c9205b281a6 Mon Sep 17 00:00:00 2001
From: Juha Reunanen <juha.reunanen@tomaattinen.com>
Date: Fri, 2 Apr 2021 20:31:04 +0300
Subject: [PATCH] Add constructor that takes generic std::istream objects (#11)

---
 README.md    | 11 +++--------
 TinyEXIF.cpp | 34 ++++++++++++++++++++++++++++++++++
 TinyEXIF.h   |  2 ++
 main.cpp     | 27 +++------------------------
 4 files changed, 42 insertions(+), 32 deletions(-)

diff --git a/README.md b/README.md
index decd06a..1d14e38 100644
--- a/README.md
+++ b/README.md
@@ -18,16 +18,11 @@ int main(int argc, const char** argv) {
 		return -1;
 	}
 
-	// read entire image file
-	std::ifstream file(argv[1], std::ifstream::in|std::ifstream::binary);
-	file.seekg(0,std::ios::end);
-	std::streampos length = file.tellg();
-	file.seekg(0,std::ios::beg);
-	std::vector<uint8_t> data(length);
-	file.read((char*)data.data(), length);
+	// open a stream to read just the necessary parts of the image file
+	std::ifstream istream(argv[1], std::ifstream::binary);
 
 	// parse image EXIF and XMP metadata
-	TinyEXIF::EXIFInfo imageEXIF(data.data(), length);
+	TinyEXIF::EXIFInfo imageEXIF(istream);
 	if (imageEXIF.Fields)
 		std::cout
 			<< "Image Description " << imageEXIF.ImageDescription << "\n"
diff --git a/TinyEXIF.cpp b/TinyEXIF.cpp
index 5943c6e..73c0086 100644
--- a/TinyEXIF.cpp
+++ b/TinyEXIF.cpp
@@ -43,6 +43,7 @@
 #include <cfloat>
 #include <vector>
 #include <algorithm>
+#include <iostream>
 
 #ifdef _MSC_VER
 namespace {
@@ -338,6 +339,9 @@ EXIFInfo::EXIFInfo() : Fields(FIELD_NA) {
 EXIFInfo::EXIFInfo(EXIFStream& stream) {
 	parseFrom(stream);
 }
+EXIFInfo::EXIFInfo(std::istream& stream) {
+	parseFrom(stream);
+}
 EXIFInfo::EXIFInfo(const uint8_t* data, unsigned length) {
 	parseFrom(data, length);
 }
@@ -848,6 +852,36 @@ int EXIFInfo::parseFrom(EXIFStream& stream) {
 	return app1s();
 }
 
+
+int EXIFInfo::parseFrom(std::istream& stream) {
+	class EXIFStdStream : public EXIFStream {
+	public:
+		EXIFStdStream(std::istream& stream)
+			: stream(stream) {
+			// Would be nice to assert here that the stream was opened in binary mode, but
+			// apparently that's not possible: https://stackoverflow.com/a/224259/19254
+		}
+		bool IsValid() const override {
+			return !!stream;
+		}
+		const uint8_t* GetBuffer(unsigned desiredLength) override {
+			buffer.resize(desiredLength);
+			if (!stream.read(reinterpret_cast<char*>(buffer.data()), desiredLength))
+				return NULL;
+			return buffer.data();
+		}
+		bool SkipBuffer(unsigned desiredLength) override {
+			return (bool)stream.seekg(desiredLength, std::ios::cur);
+		}
+	private:
+		std::istream& stream;
+		std::vector<uint8_t> buffer;
+	};
+	EXIFStdStream streamWrapper(stream);
+	return parseFrom(streamWrapper);
+}
+
+
 int EXIFInfo::parseFrom(const uint8_t* buf, unsigned len) {
 	class EXIFStreamBuffer : public EXIFStream {
 	public:
diff --git a/TinyEXIF.h b/TinyEXIF.h
index 09c26f7..4f66590 100644
--- a/TinyEXIF.h
+++ b/TinyEXIF.h
@@ -100,6 +100,7 @@ class TINYEXIF_LIB EXIFInfo {
 public:
 	EXIFInfo();
 	EXIFInfo(EXIFStream& stream);
+	EXIFInfo(std::istream& stream); // NB: the stream must have been opened in binary mode
 	EXIFInfo(const uint8_t* data, unsigned length);
 
 	// Parsing function for an entire JPEG image stream.
@@ -110,6 +111,7 @@ public:
 	// RETURN:  PARSE_SUCCESS (0) on success with 'result' filled out
 	//          error code otherwise, as defined by the PARSE_* macros
 	int parseFrom(EXIFStream& stream);
+	int parseFrom(std::istream& stream); // NB: the stream must have been opened in binary mode
 	int parseFrom(const uint8_t* data, unsigned length);
 
 	// Parsing function for an EXIF segment. This is used internally by parseFrom()
diff --git a/main.cpp b/main.cpp
index 476c476..ddd5212 100644
--- a/main.cpp
+++ b/main.cpp
@@ -9,27 +9,6 @@
 #include <vector>   // std::vector
 #include <iomanip>  // std::setprecision
 
-class EXIFStreamFile : public TinyEXIF::EXIFStream {
-public:
-	explicit EXIFStreamFile(const char* fileName)
-		: file(fileName, std::ifstream::in|std::ifstream::binary) {}
-	bool IsValid() const override {
-		return file.is_open();
-	}
-	const uint8_t* GetBuffer(unsigned desiredLength) override {
-		buffer.resize(desiredLength);
-		if (!file.read((char*)buffer.data(), desiredLength))
-			return NULL;
-		return buffer.data();
-	}
-	bool SkipBuffer(unsigned desiredLength) override {
-		return (bool)file.seekg(desiredLength,std::ios::cur);
-	}
-private:
-	std::ifstream file;
-	std::vector<uint8_t> buffer;
-};
-
 int main(int argc, const char** argv)
 {
 	if (argc != 2) {
@@ -37,9 +16,9 @@ int main(int argc, const char** argv)
 		return -1;
 	}
 
-	// read entire image file
-	EXIFStreamFile stream(argv[1]);
-	if (!stream.IsValid()) {
+	// open a stream to read just the necessary parts of the image file
+	std::ifstream stream(argv[1], std::ios::binary);
+	if (!stream) {
 		std::cout << "error: can not open '" << argv[1] << "'\n";
 		return -2;
 	}