feature: stages

stages: separate scanning and parsing, allowing `v`, `vn`, `vt` lines to
be mapped to line numbers and store line data (once).

first stage parsing: parse face data and validate vertices.

TODO:
* improve validation (detect partial normals/texcoords);
* maybe improve storage of triangle line number instead of storing line
  number for each vertex;

src/scan.hpp

@@ -0,0 +1,33 @@
+#ifndef __WAVEFRONT_FILE_HPP__
+#define __WAVEFRONT_FILE_HPP__
+
+#include <cstdint>
+#include <map>
+#include <string>
+#include <stdexcept>
+#include <vector>
+
+#include "settings.hpp"
+
+namespace wavefront {
+    class scan_error : public std::runtime_error {
+    public:
+        explicit scan_error(const std::string &message);
+    };
+
+    struct scan_result {
+        std::size_t total_lines;
+        std::map<std::size_t, std::string> line_data;
+        std::map<std::string, std::vector<std::size_t>> category_map;
+
+        scan_result();
+    };
+
+    scan_result scan(
+        std::istream &input,
+        const std::vector<std::string> &selected_objects = {},
+        const std::vector<std::string> &selected_groups = {}
+    );
+}
+
+#endif // __WAVEFRONT_PARSER_FILE_HPP__