Reading multipage TIFFs consumes too much memory

src/Tesseract/Interop/LeptonicaApi.cs

@@ -98,6 +98,9 @@ public interface ILeptonicaApiSignatures
         [RuntimeDllImport(Constants.LeptonicaDllName, CallingConvention = CallingConvention.Cdecl, EntryPoint = "pixReadMemTiff")]
         unsafe IntPtr pixReadMemTiff(byte* data, int length, int page);
 
+        [RuntimeDllImport(Constants.LeptonicaDllName, CallingConvention = CallingConvention.Cdecl, EntryPoint = "pixReadFromMultipageTiff")]
+        IntPtr pixReadFromMultipageTiff(string filename, ref int offset);
+
         [RuntimeDllImport(Constants.LeptonicaDllName, CallingConvention = CallingConvention.Cdecl, EntryPoint = "pixWrite")]
         int pixWrite(string filename, HandleRef handle, ImageFormat format);
 

src/Tesseract/Tesseract.csproj

@@ -108,6 +108,7 @@
     </Compile>
     <Compile Include="TesseractException.cs" />
     <Compile Include="TextLineOrder.cs" />
+    <Compile Include="TiffReader.cs" />
     <Compile Include="WritingDirection.cs" />
   </ItemGroup>
   <ItemGroup>

src/Tesseract/TiffReader.cs

@@ -0,0 +1,109 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+
+namespace Tesseract
+{
+    /// <summary>
+    /// Access TIFF images one at a time to limit memory utilization.
+    /// </summary>
+    public sealed class TiffReader
+    {
+        /// <summary>
+        /// The file being read.
+        /// </summary>
+        private string Filename { get; set; }
+
+        /// <summary>
+        /// IFD offset cache.
+        /// </summary>
+        private List<int> Offsets { get; set; } = new List<int>();
+
+        /// <summary>
+        /// When true EOF was encountered and there are no more IFDs.
+        /// </summary>
+        private bool reachedEndOfFile = false;
+
+        /// <summary>
+        /// Initialize an instance of TiffReader for the specified file.
+        /// </summary>
+        /// <param name="filename">The path to the TIFF file to read.</param>
+        public TiffReader(string filename)
+        {
+            if (filename == null || filename == string.Empty)
+            {
+                throw new ArgumentOutOfRangeException("filename");
+            }
+
+            Filename = filename;
+            Offsets.Add(0);
+        }
+
+        /// <summary>
+        /// Get an image corresponding to the TIFF's IFD number using the offset cache.  If the offset has not
+        /// been previously visited then the request is rejected.
+        /// </summary>
+        /// <param name="i">The IFD number, origin zero.</param>
+        /// <returns>null if the IFD isn't present, or the image.</returns>
+        public Pix GetPix(int i)
+        {
+            int offset = 0;
+            if (i < Offsets.Count)
+            {
+                offset = Offsets[i];
+            }
+            else
+            {
+                throw new IOException("Invalid sequence.");
+            }
+
+            IntPtr handle = Interop.LeptonicaApi.Native.pixReadFromMultipageTiff(Filename, ref offset);
+
+            if (handle == IntPtr.Zero)
+            {
+                return null;
+            }
+
+            return Pix.Create(handle);
+        }
+
+        /// <summary>
+        /// Get the next IFD entry from this TIFF.
+        /// </summary>
+        /// <returns>null if there are no more images.</returns>
+        public Pix NextPix()
+        {
+            if (reachedEndOfFile)
+            {
+                return null;
+            }
+
+            IntPtr handle;
+            if (Offsets.Count > 0)
+            {
+                int offset = Offsets[Offsets.Count - 1];
+                handle = Interop.LeptonicaApi.Native.pixReadFromMultipageTiff(Filename, ref offset);
+
+                if (handle == IntPtr.Zero)
+                {
+                    return null;
+                }
+
+                if (offset != 0)
+                {
+                    Offsets.Add(offset);
+                }
+                else
+                {
+                    reachedEndOfFile = true;
+                }
+            }
+            else
+            {
+                throw new IOException("Invalid state.");
+            }
+
+            return Pix.Create(handle);
+        }
+    }
+}