Add MFile::relativize method

cdm/core/src/main/java/thredds/filesystem/MFileOS.java

@@ -146,4 +146,12 @@ public File getFile() {
   public MFileOS getChild(String newFilename) {
     return new MFileOS(new File(file, newFilename));
   }
+
+  @Override
+  public String relativize(MFile other) {
+    if (other instanceof MFileOS) {
+      return file.toPath().relativize(((MFileOS) other).getFile().toPath()).toString();
+    }
+    throw new IllegalArgumentException("Cannot relativize " + other + " against " + this);
+  }
 }

cdm/core/src/main/java/thredds/filesystem/MFileOS7.java

@@ -149,4 +149,12 @@ public MFileOS7 getChild(String newFilename) {
   public Path getNioPath() {
     return path;
   }
+
+  @Override
+  public String relativize(MFile other) {
+    if (other instanceof MFileOS7) {
+      return path.relativize(((MFileOS7) other).path).toString();
+    }
+    throw new IllegalArgumentException("Cannot relativize " + other + " against " + this);
+  }
 }

cdm/core/src/main/java/thredds/inventory/CollectionManagerCatalog.java

@@ -173,6 +173,11 @@ public void writeToStream(OutputStream outputStream, long offset, long maxBytes)
     public MFileRemote getChild(String newFilename) {
       throw new UnsupportedOperationException("MFileRemote::getChild not implemented. Filename: " + getName());
     }
+
+    @Override
+    public String relativize(MFile other) {
+      throw new UnsupportedOperationException("MFileRemote::relativize not implemented. Filename: " + getName());
+    }
   }
 
   ///////////////////////////////

cdm/core/src/main/java/thredds/inventory/MFile.java

@@ -114,4 +114,12 @@ default boolean isReadable() {
    */
   @Nullable
   MFile getChild(String newFileName);
+
+  /**
+   * Construct a relative path between this MFile and a given MFile.
+   *
+   * @param other the MFile to relativize against this MFile's path
+   * @return the resulting relative path as a String, or an empty path if both paths are equal
+   */
+  String relativize(MFile other);
 }

cdm/s3/src/main/java/thredds/inventory/s3/MFileS3.java

@@ -7,6 +7,7 @@
 import java.io.IOException;
 import java.io.OutputStream;
 import java.net.URISyntaxException;
+import java.nio.file.Paths;
 import java.util.Objects;
 import java.util.function.Supplier;
 import javax.annotation.Nonnull;
@@ -382,6 +383,31 @@ public MFileS3 getChild(String newFilename) {
     }
   }
 
+  /**
+   * Construct a relative key path (as if it were a file path) between this MFile and a given MFile.
+   * They must have same delimiter and bucket, otherwise the path returned is empty.
+   *
+   * @param other the MFile to relativize against this MFile's path
+   * @return the resulting relative path as a String, or an empty path if both paths are equal
+   */
+  @Override
+  public String relativize(MFile other) {
+    if (!(other instanceof MFileS3)) {
+      throw new IllegalArgumentException("Cannot relativize " + other + " against " + this);
+    }
+    final MFileS3 otherS3 = (MFileS3) other;
+
+    if (getDelimiter() != null && getDelimiter().equals("/") && getDelimiter().equals(otherS3.getDelimiter())
+        && cdmS3Uri.getBucket().equals(otherS3.cdmS3Uri.getBucket())) {
+      final String key = getKey();
+      final String otherKey = otherS3.getKey();
+      return key == null || otherKey == null ? ""
+          : Paths.get("/" + key).relativize(Paths.get("/" + otherKey)).toString();
+    }
+
+    return "";
+  }
+
   public static class Provider implements MFileProvider {
 
     private static String protocol = CdmS3Uri.SCHEME_CDM_S3;

cdm/zarr/src/main/java/thredds/inventory/zarr/MFileZip.java

@@ -184,6 +184,11 @@ public MFileZip getChild(String newFilename) {
     throw new UnsupportedOperationException("MFileZip::getChild not implemented. Filename: " + getName());
   }
 
+  @Override
+  public String relativize(MFile other) {
+    throw new UnsupportedOperationException("MFileZip::relativize not implemented. Filename: " + getName());
+  }
+
   public Path getRootPath() {
     return rootPath;
   }

grib/src/main/java/ucar/nc2/grib/collection/GcMFile.java

@@ -141,4 +141,9 @@ public void writeToStream(OutputStream outputStream, long offset, long maxBytes)
   public GcMFile getChild(String newFilename) {
     throw new UnsupportedOperationException("GcMFile::getChild not implemented. Filename: " + getName());
   }
+
+  @Override
+  public String relativize(MFile other) {
+    throw new UnsupportedOperationException("GcMFile::relativize not implemented. Filename: " + getName());
+  }
 }