diff --git a/crates/core/src/io/file_reader.rs b/crates/core/src/io/file_reader.rs
index 936af7a..3a52fd8 100644
--- a/crates/core/src/io/file_reader.rs
+++ b/crates/core/src/io/file_reader.rs
@@ -1,13 +1,138 @@
-use crate::error::Result;
-use std::path::Path;
+use crate::error::{CoreError, Result};
+use crate::io::line_index::LineIndex;
+use std::path::{Path, PathBuf};
 
-pub struct FileReader {/* TODO */}
+pub struct FileReader {
+    path: PathBuf,
+    data: Vec<u8>,
+    line_index: LineIndex,
+}
 
 impl FileReader {
-    pub fn open(_path: &Path) -> Result<Self> {
-        todo!()
+    pub fn open(path: &Path) -> Result<Self> {
+        let data = std::fs::read(path)?;
+
+        if std::str::from_utf8(&data).is_err() {
+            return Err(CoreError::Encoding {
+                line: 0,
+                bytes: data.iter().take(64).copied().collect(),
+            });
+        }
+
+        let line_index = LineIndex::from_bytes(&data);
+
+        Ok(FileReader {
+            path: path.to_path_buf(),
+            data,
+            line_index,
+        })
     }
+
+    pub fn data(&self) -> &[u8] {
+        &self.data
+    }
+
+    pub fn line_count(&self) -> usize {
+        self.line_index.line_count()
+    }
+
+    pub fn get_line(&self, idx: usize) -> Option<&str> {
+        self.line_index.get_line(&self.data, idx)
+    }
+
     pub fn file_size(&self) -> u64 {
-        todo!()
+        self.data.len() as u64
+    }
+
+    pub fn path(&self) -> &Path {
+        &self.path
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::env::temp_dir;
+
+    fn temp_path(name: &str) -> PathBuf {
+        temp_dir().join(name)
+    }
+
+    #[test]
+    fn test_empty_file() {
+        let path = temp_path("file_reader_test_empty");
+        std::fs::write(&path, b"").unwrap();
+        let reader = FileReader::open(&path).unwrap();
+        assert_eq!(reader.line_count(), 0);
+        let _ = std::fs::remove_file(&path);
+    }
+
+    #[test]
+    fn test_multi_line_file() {
+        let path = temp_path("file_reader_test_multi");
+        std::fs::write(&path, b"hello\nworld\nfoo").unwrap();
+        let reader = FileReader::open(&path).unwrap();
+        assert_eq!(reader.line_count(), 3);
+        assert_eq!(reader.get_line(0), Some("hello"));
+        assert_eq!(reader.get_line(1), Some("world"));
+        assert_eq!(reader.get_line(2), Some("foo"));
+        let _ = std::fs::remove_file(&path);
+    }
+
+    #[test]
+    fn test_nonexistent_file() {
+        let path = temp_path("file_reader_test_nonexistent_xyzzy");
+        let _ = std::fs::remove_file(&path);
+        let result = FileReader::open(&path);
+        match result {
+            Err(CoreError::Io { .. }) => {}
+            Err(other) => panic!("expected Io variant, got {other:?}"),
+            Ok(_) => panic!("expected error, got success"),
+        }
+    }
+
+    #[test]
+    fn test_non_utf8_file() {
+        let path = temp_path("file_reader_test_nonutf8");
+        std::fs::write(&path, &[0xFF, 0xFE]).unwrap();
+        let result = FileReader::open(&path);
+        match result {
+            Err(CoreError::Encoding { line, bytes }) => {
+                assert_eq!(line, 0);
+                assert_eq!(bytes, vec![0xFF, 0xFE]);
+            }
+            Err(other) => panic!("expected Encoding variant, got {other:?}"),
+            Ok(_) => panic!("expected error, got success"),
+        }
+        let _ = std::fs::remove_file(&path);
+    }
+
+    #[test]
+    fn test_file_size() {
+        let path = temp_path("file_reader_test_size");
+        let content = b"hello world";
+        std::fs::write(&path, content).unwrap();
+        let reader = FileReader::open(&path).unwrap();
+        assert_eq!(reader.file_size(), content.len() as u64);
+        let _ = std::fs::remove_file(&path);
+    }
+
+    #[test]
+    fn test_path() {
+        let path = temp_path("file_reader_test_path");
+        std::fs::write(&path, b"data").unwrap();
+        let reader = FileReader::open(&path).unwrap();
+        assert_eq!(reader.path(), path);
+        let _ = std::fs::remove_file(&path);
+    }
+
+    #[test]
+    fn test_single_line_no_newline() {
+        let path = temp_path("file_reader_test_single");
+        std::fs::write(&path, b"hello").unwrap();
+        let reader = FileReader::open(&path).unwrap();
+        assert_eq!(reader.line_count(), 1);
+        assert_eq!(reader.get_line(0), Some("hello"));
+        let _ = std::fs::remove_file(&path);
     }
 }
diff --git a/crates/core/src/io/line_index.rs b/crates/core/src/io/line_index.rs
index 099a1e8..edca9b9 100644
--- a/crates/core/src/io/line_index.rs
+++ b/crates/core/src/io/line_index.rs
@@ -1 +1,140 @@
-pub struct LineIndex {/* TODO */}
+pub struct LineIndex {
+    line_starts: Vec<usize>,
+    #[allow(dead_code)]
+    has_trailing_newline: bool,
+}
+
+impl LineIndex {
+    pub fn from_bytes(data: &[u8]) -> Self {
+        if data.is_empty() {
+            return LineIndex {
+                line_starts: vec![],
+                has_trailing_newline: false,
+            };
+        }
+
+        let mut line_starts = vec![0usize];
+        for pos in memchr::memchr_iter(b'\n', data) {
+            line_starts.push(pos + 1);
+        }
+
+        let trailing = data.last().is_some_and(|&b| b == b'\n');
+        if trailing {
+            line_starts.pop();
+        }
+
+        LineIndex {
+            line_starts,
+            has_trailing_newline: trailing,
+        }
+    }
+
+    pub fn line_count(&self) -> usize {
+        self.line_starts.len()
+    }
+
+    pub fn get_line<'a>(&self, data: &'a [u8], idx: usize) -> Option<&'a str> {
+        if idx >= self.line_starts.len() {
+            return None;
+        }
+
+        let start = self.line_starts[idx];
+        let end = if idx + 1 < self.line_starts.len() {
+            self.line_starts[idx + 1].saturating_sub(1)
+        } else {
+            data.len()
+        };
+
+        let slice = &data[start..end];
+        std::str::from_utf8(slice)
+            .map(|s| s.trim_end_matches(['\r', '\n']))
+            .ok()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_empty_data() {
+        let idx = LineIndex::from_bytes(b"");
+        assert_eq!(idx.line_count(), 0);
+    }
+
+    #[test]
+    fn test_single_line_no_newline() {
+        let data = b"hello";
+        let idx = LineIndex::from_bytes(data);
+        assert_eq!(idx.line_count(), 1);
+        assert_eq!(idx.get_line(data, 0), Some("hello"));
+    }
+
+    #[test]
+    fn test_single_line_with_newline() {
+        let data = b"hello\n";
+        let idx = LineIndex::from_bytes(data);
+        assert_eq!(idx.line_count(), 1);
+        assert_eq!(idx.get_line(data, 0), Some("hello"));
+    }
+
+    #[test]
+    fn test_multi_line() {
+        let data = b"aaa\nbbb\nccc";
+        let idx = LineIndex::from_bytes(data);
+        assert_eq!(idx.line_count(), 3);
+        assert_eq!(idx.get_line(data, 0), Some("aaa"));
+        assert_eq!(idx.get_line(data, 1), Some("bbb"));
+        assert_eq!(idx.get_line(data, 2), Some("ccc"));
+    }
+
+    #[test]
+    fn test_crlf_endings() {
+        let data = b"hello\r\nworld\r\n";
+        let idx = LineIndex::from_bytes(data);
+        assert_eq!(idx.line_count(), 2);
+        assert_eq!(idx.get_line(data, 0), Some("hello"));
+        assert_eq!(idx.get_line(data, 1), Some("world"));
+    }
+
+    #[test]
+    fn test_only_newline() {
+        let data = b"\n";
+        let idx = LineIndex::from_bytes(data);
+        assert_eq!(idx.line_count(), 1);
+        assert_eq!(idx.get_line(data, 0), Some(""));
+    }
+
+    #[test]
+    fn test_consecutive_newlines() {
+        let data = b"a\n\nb";
+        let idx = LineIndex::from_bytes(data);
+        assert_eq!(idx.line_count(), 3);
+        assert_eq!(idx.get_line(data, 0), Some("a"));
+        assert_eq!(idx.get_line(data, 1), Some(""));
+        assert_eq!(idx.get_line(data, 2), Some("b"));
+    }
+
+    #[test]
+    fn test_double_newline() {
+        let data = b"\n\n";
+        let idx = LineIndex::from_bytes(data);
+        assert_eq!(idx.line_count(), 2);
+        assert_eq!(idx.get_line(data, 0), Some(""));
+        assert_eq!(idx.get_line(data, 1), Some(""));
+    }
+
+    #[test]
+    fn test_out_of_bounds() {
+        let data = b"hello";
+        let idx = LineIndex::from_bytes(data);
+        assert_eq!(idx.get_line(data, 999), None);
+    }
+
+    #[test]
+    fn test_empty_file_line_count_and_get_line() {
+        let idx = LineIndex::from_bytes(b"");
+        assert_eq!(idx.line_count(), 0);
+        assert_eq!(idx.get_line(b"", 0), None);
+    }
+}