sort car urls (#172)

* sort car urls

* fix

* refactor

* rename

* add size to return struct

* sort cars test

* urls

* fix reading

* add car file fixtures

* more tests

* use existing func

* cr

cmd-car-split.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"io"
 	"io/fs"
+	"net/http"
 	"os"
 	"path/filepath"
 	"sort"
@@ -44,7 +45,8 @@ var (
 		Roots:   []cid.Cid{CBOR_SHA256_DUMMY_CID}, // placeholder
 		Version: 1,
 	}
-	hdrSize, _ = car.HeaderSize(hdr)
+	hdrSize, _     = car.HeaderSize(hdr)
+	maxSectionSize = 2 << 20 // 2 MiB
 )
 
 const maxLinks = 432000 / 18 // 18 subsets
@@ -532,11 +534,13 @@ func readHeader(streamBuf *bufio.Reader) ([]byte, int64, error) {
 	return headerBuf.Bytes(), streamLen, nil
 }
 
-func SortCarFiles(carFiles []string) ([]string, error) {
-	type carFileInfo struct {
-		path      string
-		firstSlot int64
-	}
+type carFileInfo struct {
+	name      string
+	firstSlot int64
+	size      int64
+}
+
+func SortCarFiles(carFiles []string) ([]carFileInfo, error) {
 
 	var fileInfos []carFileInfo
 
@@ -584,9 +588,15 @@ func SortCarFiles(carFiles []string) ([]string, error) {
 			return nil, fmt.Errorf("failed to find root node in file %s", path)
 		}
 
+		fi, err := file.Stat()
+		if err != nil {
+			return nil, fmt.Errorf("failed to get file info for %s: %w", path, err)
+		}
+
 		fileInfos = append(fileInfos, carFileInfo{
-			path:      path,
+			name:      path,
 			firstSlot: int64(subset.First),
+			size:      fi.Size(),
 		})
 	}
 
@@ -595,11 +605,139 @@ func SortCarFiles(carFiles []string) ([]string, error) {
 		return fileInfos[i].firstSlot < fileInfos[j].firstSlot
 	})
 
-	// Extract the sorted file paths
-	sortedFiles := make([]string, len(fileInfos))
-	for i, info := range fileInfos {
-		sortedFiles[i] = info.path
+	return fileInfos, nil
+}
+
+func SortCarURLs(carURLs []string) ([]carFileInfo, error) {
+	var urlInfos []carFileInfo
+
+	for _, url := range carURLs {
+		firstSlot, size, err := getSlotAndSizeFromURL(url)
+		if err != nil {
+			return nil, fmt.Errorf("failed to get first slot from URL %s: %w", url, err)
+		}
+
+		urlInfos = append(urlInfos, carFileInfo{
+			name:      url,
+			firstSlot: firstSlot,
+			size:      size,
+		})
+	}
+
+	// Sort the URL infos based on the firstSlot
+	sort.Slice(urlInfos, func(i, j int) bool {
+		return urlInfos[i].firstSlot < urlInfos[j].firstSlot
+	})
+
+	return urlInfos, nil
+}
+
+func getSlotAndSizeFromURL(url string) (int64, int64, error) {
+	fileSize, err := splitcarfetcher.GetContentSizeWithHeadOrZeroRange(url)
+	if err != nil {
+		return 0, 0, fmt.Errorf("failed to get file size: %w", err)
+	}
+
+	rootCID, err := getRootCid(url)
+	if err != nil {
+		return 0, 0, fmt.Errorf("failed to get root CID: %w", err)
+	}
+
+	endOffset := getEndOffset(fileSize)
+
+	partialContent, err := fetchFromOffset(url, endOffset)
+	if err != nil {
+		return 0, 0, fmt.Errorf("failed to fetch partial content: %w", err)
+	}
+
+	cidBytes := rootCID.Bytes()
+	index := bytes.LastIndex(partialContent, cidBytes)
+	if index == -1 {
+		return 0, 0, fmt.Errorf("CID block not found in the last 2MiB of the file")
+	}
+	blockData := partialContent[index-2:]
+	r := bufio.NewReader(bytes.NewBuffer(blockData))
+	cid, _, data, err := carreader.ReadNodeInfoWithData(r)
+	if err != nil {
+		return 0, 0, fmt.Errorf("failed to read node info: %w", err)
+	}
+	if cid != rootCID {
+		return 0, 0, fmt.Errorf("expected CID %s, got %s", rootCID, cid)
+	}
+
+	// Decode the Subset
+	subset, err := iplddecoders.DecodeSubset(data)
+	if err != nil {
+		return 0, 0, fmt.Errorf("failed to decode Subset from block: %w", err)
+	}
+
+	return int64(subset.First), fileSize, nil
+}
+
+func getRootCid(url string) (cid.Cid, error) {
+	// Make a GET request for the beginning of the file
+	req, err := http.NewRequest("GET", url, nil)
+	if err != nil {
+		return cid.Undef, fmt.Errorf("failed to create request: %w", err)
+	}
+
+	// Request only the first hdrSize bytes
+	req.Header.Set("Range", fmt.Sprintf("bytes=0-%d", hdrSize))
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return cid.Undef, fmt.Errorf("failed to fetch CAR file header: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusPartialContent {
+		return cid.Undef, fmt.Errorf("server does not support range requests")
+	}
+
+	// Read the header content
+	headerContent, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return cid.Undef, fmt.Errorf("failed to read header content: %w", err)
+	}
+
+	// Parse the CAR header
+	rc := io.NopCloser(bytes.NewReader(headerContent))
+	cr, err := carreader.New(rc)
+	if err != nil {
+		return cid.Undef, fmt.Errorf("failed to create CarReader: %w", err)
+	}
+
+	roots := cr.Header.Roots
+	if len(roots) != 1 {
+		return cid.Undef, fmt.Errorf("expected 1 root CID, got %d", len(roots))
+	}
+	rootCID := roots[0]
+
+	return rootCID, nil
+}
+
+func getEndOffset(fileSize int64) int64 {
+	eo := fileSize - int64(maxSectionSize)
+	return max(eo, 0)
+}
+
+func fetchFromOffset(url string, offset int64) ([]byte, error) {
+	req, err := http.NewRequest("GET", url, nil)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create request: %w", err)
+	}
+
+	req.Header.Set("Range", fmt.Sprintf("bytes=%d-", offset))
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("failed to fetch CAR file: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusPartialContent {
+		return nil, fmt.Errorf("server does not support range requests")
 	}
 
-	return sortedFiles, nil
+	return io.ReadAll(resp.Body)
 }