feat: add AppConfig INI-based configuration system

Implements a zero-dependency INI parser for application settings with
[batch], [conversion], and [quality] sections. Includes AppConfig::load(),
write_default(), output_format(), and parsed_extensions() helpers, along
with a documented config.ini example file.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config.ini

@@ -0,0 +1,51 @@
+# photo-converter configuration
+# Lines starting with # or ; are comments.
+# Copy this file and adjust paths before running in batch mode:
+#   photo-converter --batch --config config.ini
+
+[batch]
+# Directory containing input images (RAW or standard formats)
+input_dir        = import
+
+# Directory where converted images are written
+output_dir       = output
+
+# Scan input_dir recursively into subdirectories (true/false)
+recursive        = false
+
+# Comma-separated list of file extensions to process (case-insensitive)
+file_extensions  = arw,cr2,cr3,nef,dng,orf,rw2,raf,pef,jpg,jpeg,png,tif,tiff
+
+[conversion]
+# Film type detection strategy:
+#   auto  – NegativeDetector analyses the histogram and orange mask
+#   c41   – force C-41 colour negative processing
+#   bw    – force B&W negative processing
+film_type        = auto
+
+# Output format:
+#   png16  – 16-bit PNG  (lossless, archival quality)
+#   png8   – 8-bit PNG   (lossless, smaller files)
+#   tiff16 – 16-bit TIFF (lossless, for professional editing)
+#   jpg    – 8-bit JPEG  (lossy, smallest files)
+output_format    = png16
+
+# Output bit depth (8 or 16); ignored when format is jpeg
+output_bit_depth = 16
+
+# Automatically detect and crop the film frame (true/false)
+auto_crop        = true
+
+# Apply unsharp-mask sharpening (true/false)
+sharpen          = true
+
+# Invert negative to positive (true/false)
+# Set to false if input is already a positive (slide/print)
+invert           = true
+
+[quality]
+# JPEG output quality [0-100]; only used when output_format = jpg
+jpeg_quality     = 95
+
+# Unsharp-mask strength [0.0-1.0]
+sharpen_strength = 0.5

src/config/AppConfig.cpp

@@ -0,0 +1,210 @@
+#include "AppConfig.h"
+#include "../converter/pipeline/Error.h"
+
+#include <algorithm>
+#include <cctype>
+#include <format>
+#include <fstream>
+#include <sstream>
+#include <string>
+
+namespace photoconv {
+
+// ─────────────────────────────────────────────────────────────────────────────
+// Internal helpers
+// ─────────────────────────────────────────────────────────────────────────────
+
+namespace {
+
+/** Strip leading/trailing ASCII whitespace from a string (in-place). */
+void trim(std::string& s) {
+    const auto not_space = [](unsigned char c) { return !std::isspace(c); };
+    s.erase(s.begin(), std::find_if(s.begin(), s.end(), not_space));
+    s.erase(std::find_if(s.rbegin(), s.rend(), not_space).base(), s.end());
+}
+
+/** Convert a string to lowercase (in-place). */
+void to_lower(std::string& s) {
+    std::ranges::transform(s, s.begin(),
+        [](unsigned char c) { return static_cast<char>(std::tolower(c)); });
+}
+
+/**
+ * @brief Parse a "true/false/yes/no/1/0" string to bool.
+ *
+ * @param v Trimmed, lowercase value string.
+ * @return true for "true", "yes", "1"; false otherwise.
+ */
+[[nodiscard]] bool parse_bool(const std::string& v) {
+    return v == "true" || v == "yes" || v == "1";
+}
+
+} // anonymous namespace
+
+// ─────────────────────────────────────────────────────────────────────────────
+// AppConfig::load
+// ─────────────────────────────────────────────────────────────────────────────
+
+std::expected<AppConfig, Error> AppConfig::load(const std::filesystem::path& path) {
+    if (!std::filesystem::exists(path)) {
+        return std::unexpected(make_error(
+            ErrorCode::FileNotFound,
+            std::format("Config file not found: {}", path.string())));
+    }
+
+    std::ifstream file{path};
+    if (!file.is_open()) {
+        return std::unexpected(make_error(
+            ErrorCode::FileReadError,
+            std::format("Cannot open config file: {}", path.string())));
+    }
+
+    AppConfig cfg{};
+    std::string current_section;
+    std::string line;
+    int line_number = 0;
+
+    while (std::getline(file, line)) {
+        ++line_number;
+
+        // Remove inline comments
+        if (const auto comment_pos = line.find_first_of("#;");
+            comment_pos != std::string::npos) {
+            line.erase(comment_pos);
+        }
+
+        trim(line);
+        if (line.empty()) continue;
+
+        // Section header: [section_name]
+        if (line.front() == '[' && line.back() == ']') {
+            current_section = line.substr(1, line.size() - 2);
+            trim(current_section);
+            to_lower(current_section);
+            continue;
+        }
+
+        // Key=Value pair
+        const auto eq_pos = line.find('=');
+        if (eq_pos == std::string::npos) {
+            // Malformed line – skip silently (non-fatal)
+            continue;
+        }
+
+        std::string key   = line.substr(0, eq_pos);
+        std::string value = line.substr(eq_pos + 1);
+        trim(key);
+        trim(value);
+        to_lower(key);
+
+        // Dispatch by section
+        if (current_section == "batch") {
+            if      (key == "input_dir")        cfg.batch.input_dir  = value;
+            else if (key == "output_dir")       cfg.batch.output_dir = value;
+            else if (key == "recursive")        cfg.batch.recursive  = parse_bool(value);
+            else if (key == "file_extensions")  cfg.batch.file_extensions = value;
+        }
+        else if (current_section == "conversion") {
+            if      (key == "film_type")        cfg.conversion.film_type        = value;
+            else if (key == "output_format")    cfg.conversion.output_format    = value;
+            else if (key == "output_bit_depth") cfg.conversion.output_bit_depth = std::stoi(value);
+            else if (key == "auto_crop")        cfg.conversion.auto_crop        = parse_bool(value);
+            else if (key == "sharpen")          cfg.conversion.sharpen          = parse_bool(value);
+            else if (key == "invert")           cfg.conversion.invert           = parse_bool(value);
+        }
+        else if (current_section == "quality") {
+            if      (key == "jpeg_quality")     cfg.quality.jpeg_quality    = std::stoi(value);
+            else if (key == "sharpen_strength") cfg.quality.sharpen_strength = std::stod(value);
+        }
+        // Unknown sections are ignored silently.
+    }
+
+    return cfg;
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+// AppConfig::output_format
+// ─────────────────────────────────────────────────────────────────────────────
+
+OutputFormat AppConfig::output_format() const noexcept {
+    const auto& fmt = conversion.output_format;
+    if (fmt == "png8")   return OutputFormat::PNG_8bit;
+    if (fmt == "tiff16") return OutputFormat::TIFF_16bit;
+    if (fmt == "jpg" || fmt == "jpeg") return OutputFormat::JPEG;
+    return OutputFormat::PNG_16bit; // default / "png16"
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+// AppConfig::parsed_extensions
+// ─────────────────────────────────────────────────────────────────────────────
+
+std::vector<std::string> AppConfig::parsed_extensions() const {
+    std::vector<std::string> result;
+    std::istringstream ss{batch.file_extensions};
+    std::string token;
+
+    while (std::getline(ss, token, ',')) {
+        trim(token);
+        if (token.empty()) continue;
+
+        // Ensure the extension starts with a dot
+        if (token.front() != '.') token = '.' + token;
+        to_lower(token);
+        result.push_back(std::move(token));
+    }
+
+    return result;
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+// AppConfig::write_default
+// ─────────────────────────────────────────────────────────────────────────────
+
+std::expected<void, Error> AppConfig::write_default(const std::filesystem::path& path) {
+    std::filesystem::create_directories(path.parent_path());
+
+    std::ofstream file{path};
+    if (!file.is_open()) {
+        return std::unexpected(make_error(
+            ErrorCode::OutputWriteFailed,
+            std::format("Cannot create config file: {}", path.string())));
+    }
+
+    file <<
+        "# photo-converter configuration\n"
+        "# Lines starting with # or ; are comments.\n"
+        "\n"
+        "[batch]\n"
+        "# Directory containing input images\n"
+        "input_dir        = .\n"
+        "# Directory where converted images are written\n"
+        "output_dir       = output\n"
+        "# Scan input_dir recursively (true/false)\n"
+        "recursive        = false\n"
+        "# Comma-separated list of file extensions to process\n"
+        "file_extensions  = arw,cr2,cr3,nef,dng,orf,rw2,raf,pef,jpg,jpeg,png,tif,tiff\n"
+        "\n"
+        "[conversion]\n"
+        "# Film type: auto | c41 | bw\n"
+        "film_type        = auto\n"
+        "# Output format: png16 | png8 | tiff16 | jpg\n"
+        "output_format    = png16\n"
+        "# Output bit depth (8 or 16)\n"
+        "output_bit_depth = 16\n"
+        "# Automatically detect and crop film frame\n"
+        "auto_crop        = true\n"
+        "# Apply unsharp-mask sharpening\n"
+        "sharpen          = true\n"
+        "# Invert negative to positive\n"
+        "invert           = true\n"
+        "\n"
+        "[quality]\n"
+        "# JPEG output quality [0-100]\n"
+        "jpeg_quality     = 95\n"
+        "# Unsharp-mask strength [0.0-1.0]\n"
+        "sharpen_strength = 0.5\n";
+
+    return {};
+}
+
+} // namespace photoconv

src/config/AppConfig.h

@@ -0,0 +1,150 @@
+#pragma once
+
+#include "../converter/output/OutputWriter.h"
+
+#include <expected>
+#include <filesystem>
+#include <string>
+#include <vector>
+
+namespace photoconv {
+
+// Forward-declare Error so we don't pull in the whole Error.h transitively
+struct Error;
+
+/**
+ * @brief Batch processing configuration.
+ *
+ * Controls which files are processed and where the results go.
+ */
+struct BatchConfig {
+    /// Source directory scanned for images.
+    std::filesystem::path input_dir{"."};
+
+    /// Destination directory for converted images.
+    std::filesystem::path output_dir{"output"};
+
+    /// Whether to scan subdirectories recursively.
+    bool recursive{false};
+
+    /// Comma-separated list of extensions to process, e.g. "arw,cr2,nef".
+    std::string file_extensions{"arw,cr2,nef,dng,jpg,png"};
+};
+
+/**
+ * @brief Conversion parameters controlling the pipeline behaviour.
+ */
+struct ConversionConfig {
+    /**
+     * @brief Film type override.
+     *
+     * "auto" lets NegativeDetector decide.
+     * "c41" forces C-41 colour negative processing.
+     * "bw" forces black-and-white negative processing.
+     */
+    std::string film_type{"auto"};
+
+    /**
+     * @brief Output file format.
+     *
+     * Accepted values: "png16", "png8", "tiff16", "jpg".
+     */
+    std::string output_format{"png16"};
+
+    /// Output bit depth (8 or 16).
+    int output_bit_depth{16};
+
+    /// Enable automatic frame cropping.
+    bool auto_crop{true};
+
+    /// Enable unsharp-mask sharpening.
+    bool sharpen{true};
+
+    /// Enable negative-to-positive inversion.
+    bool invert{true};
+};
+
+/**
+ * @brief Quality tuning parameters.
+ */
+struct QualityConfig {
+    /// JPEG quality [0, 100]; only used when output_format is "jpg".
+    int jpeg_quality{95};
+
+    /// Unsharp mask strength [0.0, 1.0]; maps to CropProcessor::kSharpenStrength.
+    double sharpen_strength{0.5};
+};
+
+/**
+ * @brief Aggregated application configuration.
+ *
+ * Loaded from an INI-style configuration file.
+ *
+ * File format example:
+ * @code
+ * [batch]
+ * input_dir   = /home/user/scans
+ * output_dir  = /home/user/output
+ * recursive   = true
+ * file_extensions = arw,cr2,nef,jpg,png
+ *
+ * [conversion]
+ * film_type       = auto
+ * output_format   = png16
+ * output_bit_depth = 16
+ * auto_crop       = true
+ * sharpen         = true
+ * invert          = true
+ *
+ * [quality]
+ * jpeg_quality    = 95
+ * sharpen_strength = 0.5
+ * @endcode
+ */
+struct AppConfig {
+    BatchConfig     batch;
+    ConversionConfig conversion;
+    QualityConfig   quality;
+
+    /**
+     * @brief Load configuration from an INI-style file.
+     *
+     * Comments (#, ;) and blank lines are ignored.
+     * Keys not present in the file retain their default values.
+     *
+     * @param path Path to the configuration file.
+     * @return Parsed AppConfig on success, or Error on I/O or parse failure.
+     */
+    [[nodiscard]] static std::expected<AppConfig, Error> load(
+        const std::filesystem::path& path);
+
+    /**
+     * @brief Derive an OutputFormat enum value from conversion.output_format.
+     *
+     * @return Corresponding OutputFormat, or OutputFormat::PNG_16bit as fallback.
+     */
+    [[nodiscard]] OutputFormat output_format() const noexcept;
+
+    /**
+     * @brief Build a list of file extensions that should be processed.
+     *
+     * Extensions are returned in lowercase with a leading dot, e.g. ".arw".
+     *
+     * @return Vector of dot-prefixed, lowercased extension strings.
+     */
+    [[nodiscard]] std::vector<std::string> parsed_extensions() const;
+
+    /**
+     * @brief Write default configuration to a file.
+     *
+     * Creates the file (and any missing parent directories) and writes
+     * a documented INI with all default values.
+     *
+     * @param path Destination path.
+     * @return Error on I/O failure.
+     */
+    [[nodiscard]] static std::expected<void, Error> write_default(
+        const std::filesystem::path& path);
+};
+
+} // namespace photoconv