add column unique numbering scheme, more efficient selects

app/Tables/ColumnNumerator.php

@@ -0,0 +1,52 @@
+<?php
+
+
+namespace App\Tables;
+
+use App\Models\Row;
+use MightyPork\Utils\Utils;
+
+/**
+ * Generates a sequence of string codes for internal naming of columns
+ *
+ * Produces database-unique lowercase identifiers for created or proposed columns.
+ * Column identifiers are used in row objects to uniquely identify data even if the
+ * revision header (columns object) is modified, such as reordering, changing name,
+ * adding new column in concurrent proposals, etc.
+ *
+ * Thanks to this uniqueness, it could even be possible to compare or merge forks
+ * of the same table.
+ */
+class ColumnNumerator
+{
+    const ALPHABET = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm',
+                     'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'];
+
+    /** @var int */
+    private $next;
+    /** @var int */
+    private $last;
+
+    /**
+     * Create numerator for the given number of columns
+     */
+    public function __construct($capacity)
+    {
+        list($this->next, $this->last) = Row::allocateColIDs($capacity);
+    }
+
+    /**
+     * Get next column name, incrementing the internal state
+     *
+     * @return string
+     */
+    public function next()
+    {
+        if ($this->next > $this->last)
+            throw new \OutOfBoundsException("Column numerator has run out of allocated GCID slots");
+
+        $key = Utils::alphabetEncode($this->next, self::ALPHABET);
+        $this->next++;
+        return $key;
+    }
+}