1 files changed, 133 insertions, 0 deletions
diff --git a/format/commitgraph/read/open_chain.go b/format/commitgraph/read/open_chain.go
new file mode 100644
index 00000000..b55f3e57
--- /dev/null
+++ b/format/commitgraph/read/open_chain.go
@@ -0,0 +1,133 @@
+package read
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"os"
+	"strings"
+
+	"codeberg.org/lindenii/furgit/internal/intconv"
+	objectid "codeberg.org/lindenii/furgit/object/id"
+)
+
+func openChain(root *os.Root, algo objectid.Algorithm) (*Reader, error) {
+	chainPath := "info/commit-graphs/commit-graph-chain"
+
+	file, err := root.Open(chainPath)
+	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return nil, &MalformedError{Path: chainPath, Reason: "missing commit-graph-chain"}
+		}
+
+		return nil, err
+	}
+
+	scanner := bufio.NewScanner(file)
+	hashes := make([]string, 0)
+
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		if line == "" {
+			continue
+		}
+
+		hashes = append(hashes, line)
+	}
+
+	scanErr := scanner.Err()
+	closeErr := file.Close()
+
+	if scanErr != nil {
+		return nil, scanErr
+	}
+
+	if closeErr != nil {
+		return nil, closeErr
+	}
+
+	if len(hashes) == 0 {
+		return nil, &MalformedError{Path: chainPath, Reason: "empty chain"}
+	}
+
+	layers := make([]layer, 0, len(hashes))
+
+	var total uint32
+
+	hashVersion, err := intconv.Uint32ToUint8(algo.PackHashID())
+	if err != nil {
+		return nil, err
+	}
+
+	for i, hashHex := range hashes {
+		expectedBaseCount, err := intconv.IntToUint32(i)
+		if err != nil {
+			closeLayers(layers)
+
+			return nil, err
+		}
+
+		if len(hashHex) != algo.HexLen() {
+			closeLayers(layers)
+
+			return nil, &MalformedError{
+				Path:   chainPath,
+				Reason: fmt.Sprintf("invalid graph hash length at line %d", i+1),
+			}
+		}
+
+		relPath := fmt.Sprintf("info/commit-graphs/graph-%s.graph", hashHex)
+
+		loaded, loadErr := openLayer(root, relPath, algo)
+		if loadErr != nil {
+			closeLayers(layers)
+
+			return nil, loadErr
+		}
+
+		if loaded.baseCount != expectedBaseCount {
+			_ = loaded.close()
+
+			closeLayers(layers)
+
+			return nil, &MalformedError{
+				Path:   relPath,
+				Reason: fmt.Sprintf("BASE count %d does not match chain depth %d", loaded.baseCount, i),
+			}
+		}
+
+		validateErr := validateChainBaseHashes(algo, hashes, i, loaded)
+		if validateErr != nil {
+			_ = loaded.close()
+
+			closeLayers(layers)
+
+			return nil, validateErr
+		}
+
+		loaded.globalFrom = total
+		loaded.baseCount = expectedBaseCount
+
+		totalNext := total + loaded.numCommits
+		if totalNext < total {
+			_ = loaded.close()
+
+			closeLayers(layers)
+
+			return nil, &MalformedError{Path: relPath, Reason: "total commit count overflow"}
+		}
+
+		total = totalNext
+
+		layers = append(layers, *loaded)
+	}
+
+	out := &Reader{
+		algo:        algo,
+		hashVersion: hashVersion,
+		layers:      layers,
+		total:       total,
+	}
+
+	return out, nil
+}