diff --git a/src/hb-ot-shape-complex-thai.cc b/src/hb-ot-shape-complex-thai.cc
index 0736ecaac..87fe5219f 100644
--- a/src/hb-ot-shape-complex-thai.cc
+++ b/src/hb-ot-shape-complex-thai.cc
@@ -29,13 +29,230 @@
 
 /* Thai / Lao shaper */
 
+
+/* PUA shaping */
+
+
+enum thai_consonant_type_t
+{
+  NC,
+  AC,
+  RC,
+  DC,
+  NOT_CONSONANT,
+  NUM_CONSONANT_TYPES = NOT_CONSONANT
+};
+
+static thai_consonant_type_t
+get_consonant_type (hb_codepoint_t u)
+{
+  if (u == 0x0E1B || u == 0x0E1D || u == 0x0E1F || u == 0x0E2C)
+    return AC;
+  if (u == 0x0E0D || u == 0x0E10)
+    return RC;
+  if (u == 0x0E0E || u == 0x0E0F)
+    return DC;
+  if (hb_in_range<hb_codepoint_t> (u, 0x0E01, 0x0E2E))
+    return NC;
+  return NOT_CONSONANT;
+}
+
+
+enum thai_mark_type_t
+{
+  AV,
+  BV,
+  T,
+  NOT_MARK,
+  NUM_MARK_TYPES = NOT_MARK
+};
+
+static thai_mark_type_t
+get_mark_type (hb_codepoint_t u)
+{
+  if (u == 0x0E31 || hb_in_range<hb_codepoint_t> (u, 0x0E34, 0x0E37) ||
+      u == 0x0E47 || hb_in_range<hb_codepoint_t> (u, 0x0E4D, 0x0E4E))
+    return AV;
+  if (hb_in_range<hb_codepoint_t> (u, 0x0E38, 0x0E3A))
+    return BV;
+  if (hb_in_range<hb_codepoint_t> (u, 0x0E48, 0x0E4C))
+    return T;
+  return NOT_MARK;
+}
+
+
+enum thai_action_t
+{
+  NOP,
+  SD,  /* Shift combining-mark down */
+  SL,  /* Shift combining-mark left */
+  SDL, /* Shift combining-mark down-left */
+  RD   /* Remove descender from base */
+};
+
+static hb_codepoint_t
+thai_pua_shape (hb_codepoint_t u, thai_action_t action, hb_font_t *font)
+{
+  struct thai_pua_mapping_t {
+    hb_codepoint_t u;
+    hb_codepoint_t win_pua;
+    hb_codepoint_t mac_pua;
+  } const *pua_mappings = NULL;
+  static const thai_pua_mapping_t SD_mappings[] = {
+    {0x0E48, 0xF70A, 0xF88B}, /* MAI EK */
+    {0x0E49, 0xF70B, 0xF88E}, /* MAI THO */
+    {0x0E4A, 0xF70C, 0xF891}, /* MAI TRI */
+    {0x0E4B, 0xF70D, 0xF894}, /* MAI CHATTAWA */
+    {0x0E4C, 0xF70E, 0xF897}, /* THANTHAKHAT */
+    {0x0E38, 0xF718, 0xF89B}, /* SARA U */
+    {0x0E39, 0xF719, 0xF89C}, /* SARA UU */
+    {0x0E3A, 0xF71A, 0xF89D}, /* PHINTHU */
+    {0x0000, 0x0000, 0x0000}
+  };
+  static const thai_pua_mapping_t SDL_mappings[] = {
+    {0x0E48, 0xF705, 0xF88C}, /* MAI EK */
+    {0x0E49, 0xF706, 0xF88F}, /* MAI THO */
+    {0x0E4A, 0xF707, 0xF892}, /* MAI TRI */
+    {0x0E4B, 0xF708, 0xF895}, /* MAI CHATTAWA */
+    {0x0E4C, 0xF709, 0xF898}, /* THANTHAKHAT */
+    {0x0000, 0x0000, 0x0000}
+  };
+  static const thai_pua_mapping_t SL_mappings[] = {
+    {0x0E48, 0xF713, 0xF88A}, /* MAI EK */
+    {0x0E49, 0xF714, 0xF88D}, /* MAI THO */
+    {0x0E4A, 0xF715, 0xF890}, /* MAI TRI */
+    {0x0E4B, 0xF716, 0xF893}, /* MAI CHATTAWA */
+    {0x0E4C, 0xF717, 0xF896}, /* THANTHAKHAT */
+    {0x0E31, 0xF710, 0xF884}, /* MAI HAN-AKAT */
+    {0x0E34, 0xF701, 0xF885}, /* SARA I */
+    {0x0E35, 0xF702, 0xF886}, /* SARA II */
+    {0x0E36, 0xF703, 0xF887}, /* SARA UE */
+    {0x0E37, 0xF704, 0xF888}, /* SARA UEE */
+    {0x0E47, 0xF712, 0xF889}, /* MAITAIKHU */
+    {0x0E4D, 0xF711, 0xF899}, /* NIKHAHIT */
+    {0x0000, 0x0000, 0x0000}
+  };
+  static const thai_pua_mapping_t RD_mappings[] = {
+    {0x0E0D, 0xF70F, 0xF89A}, /* YO YING */
+    {0x0E10, 0xF700, 0xF89E}, /* THO THAN */
+    {0x0000, 0x0000, 0x0000}
+  };
+
+  switch (action) {
+    default: assert (false); /* Fallthrough */
+    case NOP: return u;
+    case SD:  pua_mappings = SD_mappings; break;
+    case SDL: pua_mappings = SDL_mappings; break;
+    case SL:  pua_mappings = SL_mappings; break;
+    case RD:  pua_mappings = RD_mappings; break;
+  }
+  for (; pua_mappings->u; pua_mappings++)
+    if (pua_mappings->u == u)
+    {
+      hb_codepoint_t glyph;
+      if (hb_font_get_glyph (font, pua_mappings->win_pua, 0, &glyph))
+	return pua_mappings->win_pua;
+      if (hb_font_get_glyph (font, pua_mappings->mac_pua, 0, &glyph))
+	return pua_mappings->mac_pua;
+      break;
+    }
+  return u;
+}
+
+
+static enum thai_above_state_t
+{     /* Cluster above looks like: */
+  T0, /*  ⣤                      */
+  T1, /*     ⣼                   */
+  T2, /*        ⣾                */
+  T3, /*           ⣿             */
+  NUM_ABOVE_STATES
+} thai_above_start_state[NUM_CONSONANT_TYPES + 1/* For NOT_CONSONANT */] =
+{
+  T0, /* NC */
+  T1, /* AC */
+  T0, /* RC */
+  T0, /* DC */
+  T3, /* NOT_CONSONANT */
+};
+
+static const struct thai_above_state_machine_edge_t {
+  thai_action_t action;
+  thai_above_state_t next_state;
+} thai_above_state_machine[NUM_ABOVE_STATES][NUM_MARK_TYPES] =
+{        /*AV*/    /*BV*/    /*T*/
+/*T0*/ {{NOP,T3}, {NOP,T0}, {SD, T3}},
+/*T1*/ {{SL, T2}, {NOP,T1}, {SDL,T2}},
+/*T2*/ {{NOP,T3}, {NOP,T2}, {SL, T3}},
+/*T3*/ {{NOP,T3}, {NOP,T3}, {NOP,T3}},
+};
+
+
+static enum thai_below_state_t
+{
+  B0, /* No descender */
+  B1, /* Removable descender */
+  B2, /* Strict descender */
+  NUM_BELOW_STATES
+} thai_below_start_state[NUM_CONSONANT_TYPES + 1/* For NOT_CONSONANT */] =
+{
+  B0, /* NC */
+  B0, /* AC */
+  B1, /* RC */
+  B2, /* DC */
+  B2, /* NOT_CONSONANT */
+};
+
+static const struct thai_below_state_machine_edge_t {
+  thai_action_t action;
+  thai_below_state_t next_state;
+} thai_below_state_machine[NUM_BELOW_STATES][NUM_MARK_TYPES] =
+{        /*AV*/    /*BV*/    /*T*/
+/*B0*/ {{NOP,B0}, {NOP,B2}, {NOP, B0}},
+/*B1*/ {{NOP,B1}, {RD, B2}, {NOP, B1}},
+/*B2*/ {{NOP,B2}, {SD, B2}, {NOP, B2}},
+};
+
+
 static void
 do_thai_pua_shaping (const hb_ot_shape_plan_t *plan,
 		     hb_buffer_t              *buffer,
 		     hb_font_t                *font)
 {
+  thai_above_state_t above_state = thai_above_start_state[NOT_CONSONANT];
+  thai_below_state_t below_state = thai_below_start_state[NOT_CONSONANT];
+  unsigned int base = 0;
+
+  hb_glyph_info_t *info = buffer->info;
+  unsigned int count = buffer->len;
+  for (unsigned int i = 0; i < count; i++)
+  {
+    thai_mark_type_t mt = get_mark_type (info[i].codepoint);
+
+    if (mt == NOT_MARK) {
+      thai_consonant_type_t ct = get_consonant_type (info[i].codepoint);
+      above_state = thai_above_start_state[ct];
+      below_state = thai_below_start_state[ct];
+      base = i;
+      continue;
+    }
+
+    const thai_above_state_machine_edge_t &above_edge = thai_above_state_machine[above_state][mt];
+    const thai_below_state_machine_edge_t &below_edge = thai_below_state_machine[below_state][mt];
+    above_state = above_edge.next_state;
+    below_state = below_edge.next_state;
+
+    /* At least one of the above/below actions is NOP. */
+    thai_action_t action = above_edge.action != NOP ? above_edge.action : below_edge.action;
+
+    if (action == RD)
+      info[base].codepoint = thai_pua_shape (info[base].codepoint, action, font);
+    else
+      info[i].codepoint = thai_pua_shape (info[i].codepoint, action, font);
+  }
 }
 
+
 static void
 preprocess_text_thai (const hb_ot_shape_plan_t *plan,
 		      hb_buffer_t              *buffer,