Huarong Pass (Huarong dao 华容道) is a sliding block puzzle that’s popular throughout China. The rectangular board is a battlefield; the large square tile is Cao Cao; the other nine tiles are opposing commanders and soldiers; and the opening at the bottom of the board is Huarong Pass. Initially the tiles are arranged as shown here, with Cao Cao’s tile blocked by the other nine. The player’s job is to slide tiles horizontally and vertically so that Cao Cao can eventually escape through the pass.

Cao Cao was a shrewd and clever strategist for the Wei Kingdom during the Eastern Han dynasty (25-220). The horizontal rectangular tile directly below Cao Cao is Guan Yu, a commander in the Shu Kingdom army who had once served under Cao Cao. The four vertical rectangular tiles are named after other Shu commanders, and the four small square tiles are Shu soldiers.

In the year 208, Cao Cao led 220,000 troops of the Wei army to fight against an army of 50,000 Shu troops in a mountainous area near Chibi in today’s Hubei Province. Because of some strategic errors, Cao Cao’s troops were badly defeated in the Battle of Chibi, and he fled with only a handful of his soldiers.

A fictionalized version of Cao Cao’s escape is recounted in Three Kingdoms, the classic historical novel by Luo Guanzhong (c. 1330-1400), and is known to every schoolchild in China. According to this story, the only way for Cao Cao to escape was through the narrow Huarong Pass. However, the Shu strategist Zhuge Liang had already placed his commander Guan Yu there, and Cao Cao was captured. But taking advantage of their old friendship, Cao Cao persuaded Guan Yu to let him go.

 

History of Huarong Pass Sliding Block Puzzle

Prof. Jiang Changying (1904-2006) of Northwest Industrial University in Xi’an was the first person in China to write about the sliding block puzzle based on this story. He described it in his 1949 book Science Entertainment, and he named the puzzle Huarong Dao (Huarong Pass). Jiang believed that the puzzle was invented in the 1930s and became popular in China during the late 1930s and early 1940s.

In 1938, when the Anti-Japanese War began, Prof. Lin Dekuan of Northwestern Industrial University left his home in Hanzhong, Shaanxi Province, and went to the countryside of Chenggu County to avoid the Japanese air raids. There he saw children playing Huarong Pass puzzles made of paper.

In 1943, Liang Qing of Xi’an was a cultural teacher in the New Fourth Route Army. He learned Huarong Pass from people in northern Jiangsu Province and publicized it among the soldiers. Liang collected many different initial configurations of Huarong Pass and gave each a name for easy reference. The most common configuration was called “Blocked by Broad Sword and Standing Horses.” Other configurations were “Can’t Fly Out, Even with Wings” and “Layered Blockage.”

Many other initial configurations with descriptive names have also been created.

According to Jiang Changying, Huarong Pass probably came to Shanghai in the early 1940s from northern Jiangsu Province. In the early 1950s, street vendors sold Huarong Pass printed on paper. Later, Jiang saw a wood version sold in a Shanghai stationary store. In 1956, the puzzle was written up in a math magazine, where it was called “Guan Yu Releases Cao Cao.” In 1959, it appeared in Liaoning Pictorial with the revolutionary name “Chase Away the Paper Tiger.” In the 1960s, Shanghai toy factories made plastic versions named “Parking the Boats.” And in the 1980s, Huarong Pass enthusiasts formed an association and organized competitions in Beijing, Shanghai and Northeast China.

In 1932, J. H. Fleming of Lancashire, England, patented a sliding block puzzle very similar to Huarong Pass, with the same tiles, board and escape route at the bottom. Versions of the puzzle appeared in France, Japan and other countries—each with its own theme and characters. The minimum-move solution—requiring 81 moves—was worked out by Thomas B. Lemann of New Orleans and was published in the March 1964 issue of Scientific American.

Regardless of its origin, the mapping of the classic story of Cao Cao’s capture and escape onto this sliding block puzzle has made Huarong Pass uniquely Chinese.

 

Solving Huarong Pass

Now it’s time for you to try your hand at Huarong Pass. You can make your own set out of wood or cardboard, or you can purchase it from a puzzle shop. You can even play an online version. Initially you shouldn’t worry about how many moves you take to free Cao Cao. Just try your own strategies until you find one that works. If you get frustrated you can always remove the tiles and start again.

If you would like to help Cao Cao to escape through Huarong Pass in 81 moves, here are a few intermediate steps to guide you along the way. Notice that Cao Cao and the opposing commanders on his left and right do not budge for the first 24 moves, so all the initial action takes place on the lower part of the battleground. Continue to follow the indicated steps, and lead Cao Cao through the opening at the base on the 81st move.

After you’ve led Cao Cao through Huarong Pass, try to do it again without looking at the hints. And don’t forget to try solving the other configurations too. Good luck!

 

References

Martin Gardner. “Mathematical Games” in Scientific American, vol. 210, no. 3 (March 1964), p. 128.

Edward Hordern. Sliding Block Puzzles. Oxford, 1986.

Jiang Changying. Kexue xiaoqian (Science Entertainment). Shanghai, 1949.

Jiang Changying. Kexue siwei yu xiaoqian (Science Thought Exercise and Entertainment). Xi’an, 1997.

Luo Guanzhong (c. 1330-1400). San guo yan yi (Three Kingdoms).

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