--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/validate.go	Sun Oct 16 10:08:11 2016 +0200
@@ -0,0 +1,133 @@
+package takuzu
+
+import (
+	"fmt"
+
+	"github.com/pkg/errors"
+)
+
+// checkRange returns true if the range is completely defined, and an error
+// if it doesn't follow the rules for a takuzu line or column
+// Note that the boolean might be invalid if the error is not nil.
+func checkRange(cells []Cell) (bool, error) {
+	full := true
+	size := len(cells)
+	counters := []int{0, 0}
+
+	var prevCell Cell
+	var prevCellCount int
+
+	for _, c := range cells {
+		if !c.Defined {
+			full = false
+			prevCell.Defined = false
+			prevCellCount = 0
+			continue
+		}
+		counters[c.Value]++
+		if prevCellCount == 0 {
+			prevCellCount = 1
+		} else {
+			if c.Value == prevCell.Value {
+				prevCellCount++
+				if prevCellCount > 2 {
+					return full, errors.Errorf("3+ same values %d", c.Value)
+				}
+			} else {
+				prevCellCount = 1
+			}
+
+		}
+		prevCell = c
+	}
+	if counters[0] > size/2 {
+		return full, errors.Errorf("too many zeroes")
+	}
+	if counters[1] > size/2 {
+		return full, errors.Errorf("too many ones")
+	}
+	return full, nil
+}
+
+// CheckRangeCounts returns true if all cells of the provided range are defined,
+// as well as the number of 0s and the number of 1s in the range.
+func CheckRangeCounts(cells []Cell) (full bool, n0, n1 int) {
+	counters := []int{0, 0}
+	full = true
+
+	for _, c := range cells {
+		if c.Defined {
+			counters[c.Value]++
+		} else {
+			full = false
+		}
+	}
+	return full, counters[0], counters[1]
+}
+
+// CheckLine returns an error if the line i fails validation
+func (b Takuzu) CheckLine(i int) error {
+	_, err := checkRange(b.GetLine(i))
+	return err
+}
+
+// CheckColumn returns an error if the column i fails validation
+func (b Takuzu) CheckColumn(i int) error {
+	_, err := checkRange(b.GetColumn(i))
+	return err
+}
+
+// Validate checks a whole board for errors (not completeness)
+// Returns true if all cells are defined.
+func (b Takuzu) Validate() (bool, error) {
+	finished := true
+
+	computeVal := func(cells []Cell) (val int) {
+		for i := 0; i < len(cells); i++ {
+			val += cells[i].Value * 1 << uint(i)
+		}
+		return
+	}
+
+	lineVals := make(map[int]bool)
+	colVals := make(map[int]bool)
+
+	for i := 0; i < b.Size; i++ {
+		var d []Cell
+		var full bool
+		var err error
+
+		// Let's check line i
+		d = b.GetLine(i)
+		full, err = checkRange(d)
+		if err != nil {
+			return false, errors.Wrapf(err, "line %d", i)
+		}
+		if full {
+			hv := computeVal(d)
+			if lineVals[hv] {
+				return false, fmt.Errorf("duplicate lines (%d)", i)
+			}
+			lineVals[hv] = true
+		} else {
+			finished = false
+		}
+
+		// Let's check column i
+		d = b.GetColumn(i)
+		full, err = checkRange(d)
+		if err != nil {
+			return false, errors.Wrapf(err, "column %d", i)
+		}
+		if full {
+			hv := computeVal(d)
+			if colVals[hv] {
+				return false, fmt.Errorf("duplicate columns (%d)", i)
+			}
+			colVals[hv] = true
+		} else {
+			finished = false
+		}
+	}
+	return finished, nil
+}