Handle case when file size is too small for sample size

This fixes the case where hashing may crash when using custom parameters
if the sample size is too large for the file.

- Update the spec and code to correctly constraint mode based on file size
- Update tests
- Minor lint fixes

algorithm.md

@@ -19,25 +19,28 @@ This is done in two phases:
 
 imohash takes two parameters, as well as the message length:
 
-* sample size (s)
-* sampling threshold (t)
-* message length (L)
+- sample size (s)
+- sampling threshold (t)
+- message length (L)
 
-There are two mode of operation: **sampled** and **full**. Mode is
-determined as follows:
+There are two mode of operation: **sampled** and **full**.
+
+**Full** mode is a single hash of the entire message. While sampling is the key point of imohash, sometimes it doesn't make sense and a full hash is used. It is used when the message length is less than the sampling threshold, or is less than twice the sample size - 1 (in order to sample from the middle of the message). **Full** mode is also used when the sample size parameter is less than 1.
+
+In all other cases **sampled** mode is used. Summarized:
 
 ```
-if (s > 0) && (t > 0) && (L > t) && (t > 2s)
-  mode = sampled
-else
+if (s < 1) || (L < t) || (L < (2s - 1))
   mode = full
+else
+  mode = sampled
 ```
 
 ### Hash calculation
 
 The core hashing routine uses [MurmurHash3](https://code.google.com/p/smhasher/wiki/MurmurHash3) in a 128-bit configuration.
-Hashing in *Full* mode is identical to passing the entire
-message to Murmhash3.  *Sampled* mode constructs a new message using
+Hashing in **Full** mode is identical to passing the entire
+message to Murmhash3. **Sampled** mode constructs a new message using
 three samples from the original:
 
 Message M of length L is an array of bytes, M[0]...M[L-1]. If
@@ -51,6 +54,7 @@ S2 = M[L-s:L-1]
 
 h' = Murmur3(concat(S0, S1, S2))
 ```
+
 ### Size injection
 
 Size is inserted into the hash directly. This means that two files
@@ -119,8 +123,8 @@ threshold t.
 {16384, 131073, 131072, "808008282d3f3b53e1fd132cc51fcc1d"},
 {16384, 131072, 500000, "a0c21e44a0ba3bddee802a9d1c5332ca"},
 {50,    131072, 300000, "e0a712edd8815c606344aed13c44adcf"},
+{0,     100,    1000,  "e80753211a57ee0de67c756e98e00496"},
+{50,    9999,   1000,  "e80753211a57ee0de67c756e98e00496"},
+{501,   20,     1000,  "e80753211a57ee0de67c756e98e00496"},
+{501,   20,     1001,  "e9079899cffb46f60c8645a01f12f9c9"},
 ```
-
-
-
-

imohash.go

@@ -25,7 +25,6 @@ type ImoHash struct {
 	hasher          murmur3.Hash128
 	sampleSize      int
 	sampleThreshold int
-	bytesAdded      int
 }
 
 // New returns a new ImoHash using the default sample size
@@ -79,11 +78,10 @@ func (imo *ImoHash) Sum(data []byte) [Size]byte {
 // SumFile hashes a file using using the ImoHash parameters.
 func (imo *ImoHash) SumFile(filename string) ([Size]byte, error) {
 	f, err := os.Open(filename)
-	defer f.Close()
-
 	if err != nil {
 		return emptyArray, err
 	}
+	defer f.Close()
 
 	fi, err := f.Stat()
 	if err != nil {
@@ -104,27 +102,30 @@ func (imo *ImoHash) hashCore(f *io.SectionReader) ([Size]byte, error) {
 
 	imo.hasher.Reset()
 
-	if f.Size() < int64(imo.sampleThreshold) || imo.sampleSize < 1 {
+	msgLen := f.Size()
+	if imo.sampleSize < 1 ||
+		msgLen < int64(imo.sampleThreshold) ||
+		msgLen < int64(2*imo.sampleSize-1) {
 		if _, err := io.Copy(imo.hasher, f); err != nil {
 			return emptyArray, err
 		}
 	} else {
 		buffer := make([]byte, imo.sampleSize)
-		if _, err := f.Read(buffer); err != nil {
+		if _, err := io.ReadFull(f, buffer); err != nil {
 			return emptyArray, err
 		}
 		imo.hasher.Write(buffer) // these Writes never fail
 		if _, err := f.Seek(f.Size()/2, 0); err != nil {
 			return emptyArray, err
 		}
-		if _, err := f.Read(buffer); err != nil {
+		if _, err := io.ReadFull(f, buffer); err != nil {
 			return emptyArray, err
 		}
 		imo.hasher.Write(buffer)
 		if _, err := f.Seek(int64(-imo.sampleSize), 2); err != nil {
 			return emptyArray, err
 		}
-		if _, err := f.Read(buffer); err != nil {
+		if _, err := io.ReadFull(f, buffer); err != nil {
 			return emptyArray, err
 		}
 		imo.hasher.Write(buffer)

spec_test.go

@@ -25,12 +25,17 @@ func TestSpec(t *testing.T) {
 		{16384, 131073, 131072, "808008282d3f3b53e1fd132cc51fcc1d"},
 		{16384, 131072, 500000, "a0c21e44a0ba3bddee802a9d1c5332ca"},
 		{50, 131072, 300000, "e0a712edd8815c606344aed13c44adcf"},
+
+		{0, 100, 1000, "e80753211a57ee0de67c756e98e00496"},
+		{50, 9999, 1000, "e80753211a57ee0de67c756e98e00496"},
+		{501, 20, 1000, "e80753211a57ee0de67c756e98e00496"},
+		{501, 20, 1001, "e9079899cffb46f60c8645a01f12f9c9"},
 	}
 
 	for _, test := range tests {
 		i := NewCustom(test.s, test.t)
 		hashStr = fmt.Sprintf("%x", i.Sum(M(test.n)))
-		equal(t, hashStr, test.hash)
+		equal(t, test.hash, hashStr)
 	}
 }