1 files changed, 105 insertions, 0 deletions
diff --git a/src/main/model/asn1/parsing/BytesReader.java b/src/main/model/asn1/parsing/BytesReader.java
new file mode 100644
index 0000000..3e11ea6
--- /dev/null
+++ b/src/main/model/asn1/parsing/BytesReader.java
@@ -0,0 +1,105 @@
+package model.asn1.parsing;
+
+import model.asn1.ASN1Length;
+import model.asn1.Tag;
+import model.asn1.exceptions.ParseException;
+
+/**
+ * A mutable model represents a one-way pipe of reading the input DER bytes. It keeps track of the total input and the
+ * current location, and it provides useful methods of requiring one or more bytes to present in the next.
+ */
+public class BytesReader {
+    private final Byte[] rawInput;
+    private int index;
+
+    /**
+     * EFFECTS: Initialize the reader with the given input and index set to 0.
+     * REQUIRES: rawInput.length > 0
+     */
+    public BytesReader(Byte[] rawInput) {
+        this.rawInput = rawInput;
+        this.index = 0;
+    }
+
+    /**
+     * EFFECTS: Calculate the number of bytes remaining to read.
+     */
+    public int bytesRemaining() {
+        return rawInput.length - index;
+    }
+
+    /**
+     * EFFECTS: Copy the given number of bytes from the [getIndex(), getIndex() + size) and optionally mark as read.
+     * MODIFIES: this (if markAsRead == true)
+     * REQUIRES: size <= bytesRemaining(), size > 0
+     */
+    public Byte[] read(int size, boolean markAsRead) {
+        Byte[] result = new Byte[size];
+        System.arraycopy(rawInput, index, result, 0, size);
+        if (markAsRead) {
+            index += size;
+        }
+        return result;
+    }
+
+    /**
+     * EFFECTS: Copy the given number of bytes from [getIndex(), getIndex() + size) and optionally mark as read.
+     *   Throws {@link ParseException} if size > bytesRemaining().
+     * MODIFIES: this (if markAsRead == true)
+     * REQUIRES: size > 0
+     */
+    public Byte[] require(int size, boolean markAsRead) throws ParseException {
+        validateSize(size);
+        return read(size, markAsRead);
+    }
+
+    /**
+     * EFFECTS: Check if size <= bytesRemaining().
+     *   Throws {@link ParseException if not}.
+     * REQUIRES: size > 0
+     */
+    public void validateSize(int size) throws ParseException {
+        if (size > bytesRemaining()) {
+            throw new ParseException(String.format("%d required at location %d, but only has %d before EOF.",
+                    size,
+                    index,
+                    bytesRemaining()));
+        }
+    }
+
+    /**
+     * EFFECTS: Check if the next byte has the desired tag, without changing the index.
+     *   Throws {@link ParseException} if the input is illegal (not even a tag or EOF).
+     */
+    public boolean detectTag(Tag desired) throws ParseException {
+        final int i = index;
+        final Tag t = new Tag(this);
+        index = i;
+        return t.getCls() == desired.getCls()
+                && t.isConstructive() == desired.isConstructive()
+                && t.getNumber() == desired.getNumber();
+    }
+
+    /**
+     * EFFECTS: Get the current tag or the tag immediately following (inner) without changing the index.
+     *   Throws {@link ParseException} if the input is illegal (not even a tag or EOF).
+     */
+    public Tag getTag(boolean inner) throws ParseException {
+        final int i = index;
+        Tag t = new Tag(this);
+        if (inner) {
+            new ASN1Length(this);
+            t = new Tag(this);
+        }
+        index = i;
+        return t;
+    }
+
+    public Byte[] getRawInput() {
+        return rawInput;
+    }
+
+    public int getIndex() {
+        return index;
+    }
+}