Russell Atkinson was left with a fractured skull after drug addict Christopher Subham attacked him under the Dark Arches in Leeds city centre.
Horrified members of the public witnessed the attack as Subham used the weapon to deliver the sickening blow during rush hour on the morning of August 12 this year.
Leeds Crown Court heard the 38-year-old carried out the attack as revenge after Mr Atkinson "got the better" of Subham during a confrontation earlier the same morning.
CCTV cameras followed Subham as he went in search of a weapon before returning to the Dark Arches area and striking Mr Atkinson, 41, as he was defenceless.
The court heard the two men had been involved in an altercation at 8.45am on Bishopgate Street.
Giles Grant, prosecuting, said Mr Atkinson was with a group of men outside a takeaway.
He said: "Witnesses formed the view that they were homeless and in drink."
Subham was on his way to collect a methadone prescription from a pharmacy at Leeds Railway Station at the time of the incident.
Footage showed a "scuffle" between the two men before Subham walked away.
The defendant went to collect his prescription before approaching a building site close to the station.
Subham was filmed on security cameras as he squeezed through a fence and picked up a piece of wood which he later discarded.
He then went to another building site near to the Trinity Centre and picked up a larger piece of wood which was over a meter in length.
Subham returned to the Dark Arches and struck Mr Atkinson once to the head.
He then ran off and threw the weapon away.
The piece of wood was later recovered by police and was found to contain Subham's DNA
An ambulance was called and paramedics were immediately aware that Mr Atkinson had suffered a severe head injury.
He underwent surgery at Leeds General Infirmary for a bleed to his brain after suffering a complex skull fracture.
Mr Atkinson was induced into a coma for six weeks and kept in intensive care.
He remains in Chapel Allerton Hospital where he is undergoing rehabilitation.
The court heard it is unlikely Mr Atkinson will recover fully from his injuries.
The prosecutor said Mr Atkinson was incapable of providing a victim statement to the court.
His mother provided a statement in which she described having to tell Mr Atkinson's only child that he may not recover from his injuries.
Subham pleaded guilty to causing grievous bodily harm with intent.
He has previous convictions for robbery, theft and drug offences.
Michael Walsh, mitigating, said Subham had not been drinking and was not under the influence of drugs at the time of the incident.
He said: "He was minding his own business when he was challenged by the complainant.
"It is clear from the CCTV evidence that there was a disagreement between them.
"It was the complainant that assaulted Mr Subham."
Mr Walsh said Subham went in searched of the weapon before returning to "have it out" with Mr Atkinson.
He added: "He was still overwhelmed by the feelings of upset at being assaulted by the complainant."
Subham was given an extended sentence of 14 years after Judge Geoffrey Marson, QC, said he considered the defendant to pose a serious danger the public.
He must serve a custodial period of 10 years followed by a further four years on licence.
The judge told Subham: "Whatever had gone on, instead of putting it to one side, you decided to attack him in revenge.
"You clearly thought about that, you went to pick up your methadone and then, in premeditation, deliberately set out to find a suitable weapon with which to attack him."
After the case, Investigating office Detective Constable Ian Grice, of British Tranport Police, said: “This was a horrific and despicable act of pre-meditated violence. Subham purposefully strode up to his victim, who was sitting in a doorway, and without any hesitation smashed him over the head with the piece of wood with the nails exposed.
“It is only by pure good fortune that the victim did not die from his injuries however it is still not known if he will fully recover. His family must now deal with the ongoing recovery of their son from his horrendous injuries.”